ENGINEERING  LIBRARY 


ARMATURE  WINDING 

AND 

MOTOR  REPAIR 


ARMATURE  WINDING 

AND 

MOTOR  REPAIR 


Practical  Information  and  Data  Covering  Winding  and  Reconnecting 

Procedure  for  Direct  and  Alternating  Current  Machines,  Compiled 

for  Electrical  Men  Responsible  for  the  Operation  and  Repair 

of  Motors  and  Generators  in  Industrial  Plants  and  for 

Repairmen  and  Armature  Winders  in 

Electrical  Repair  Shops 


BY 
DANIEL  H.  BRAYMER,  A.  B.,  E.  E. 

AMERICAN  INSTITUTE  OF   ELECTRICAL  ENGINEERS — MANAGING  EDITOR 
OF  ELECTRICAL  WORLD — FORMERLY  EDITOR  OF   ELECTRICAL 
ENGINEERING   AND  OF   ELECTRICAL  RECORD 


FIRST  EDITION 
TWENTY-SEVENTH  IMPRESSION 


McGRAW-HILL  BOOK  COMPANY,  INC. 

NEW    YORK   AND   LONDON 


HfiiJl 

UttW 


ENGINEERING  LIBRARY 

COPYRIGHT,  1920,  BY  THE 
MCGRAW-HILL  BOOK  COMPANY,  INC. 


PRINTED  IN  THE  UNITED   STATES  OP  AMERICA 


THE  MAPLE  PRESS  -  VORK  PA 


PREFACE 

In  this  book  no  attempt  has  been  made  to  discuss  the  sub- 
ject of  armature  winding  from  theoretical  or  design  stand- 
points. On  the  contrary,  it  is  a  compilation  of  practical 
methods  tha't  are  used  by  repairmen  and  armature  winders. 
In  selecting  the  material  a  special  effort  has  been  made  to 
include  as  far  as  possible  details  of  those  methods  which  have 
been  found  by  actual  experience  to  represent  best  practice  in 
a  repair  shop  of  average  size.  In  this  work  the  writer  has 
drawn  from  his  own  experience  in  repair  work,  from  the  ex- 
periences of  repairmen  and  armature  winders  in  large  and  small 
repair  shops  and  manufacturing  plants  which  have  been 
visited,  from  descriptions  of  practical  methods  and  the  pro- 
cedure followed  in  the  solution  of  special  problems  as  presented 
by  practical  men  in  technical  journals. 

The  title  of  repairman  as  used  throughout  this  book  is  one 
that  a  good  engineer  can  bear  with  pride  when  he  measures 
up  to  all  its  qualifications.  Such  an  engineer  is  one  who  in 
in  the  majority  of  cases  not  only  knows  what  to  do  in  the  case 
of  an  electrical  trouble  but  just  how  to  proceed  to  do  that 
particular  thing  and  who  seldom  guesses  without  a  good  per- 
centage of  the  probabilities  of  being  right  in  his  favor.  The 
main  difference  between  the  designer  and  the  repairman  is 
that  the  former  must  know  what  to  do  while  the  latter  must 
know  how  to  do  it.  A  capable  repairman  combines  both 
qualifications  through  years  of  experience. 

When  called  upon  to  locate  troubles  in  motors  and  genera- 
tors, electricians  and  repairmen  whose  experience  in  this  kind 
of  work  has  been  limited  often  find  themselves  wondering  just 
what  to  do  first.  It  is  from  just  this  viewpoint  that  the  infor- 
mation on  winding  procedure  and  the  hunting  and  correcting 
of  troubles  has  been  presented.  That  is,  instead  of  discussing 
the  fundamentals  involved  in  any  method  of  working  out  a 
repair  problem,  the  actual  problem  or  job  as  the  case  may  be 
is  discussed  from  the  " how-to-do-it"  standpoint.  Then  for 
each  individual  operation  or  procedure  the  applications  of 


M184967 


vi  PREFACE 

fundamental  laws  and  rules  are  worked  out.  Considerable 
repetition  of  some  details  of  similar  methods  will  therefore  be 
discovered  in  connection  with  information  covering  such  pro- 
cedure as  the  rewinding  of  machines  of  the  same  class  but 
of  different  types.  This  has  been  considered  advisable  since 
a  repairman  should  not  be  required  to  study  a  complete 
volume  when  details  and  information  are  desired  at  any  one 
time  on  the  procedure  for  a  particular  type  of  winding  for  a 
particular  design  of  machine. 

Liberal  use  has  been  made  of  practical  data  and  practices 
in  repair  shops  so  as  to  combine  the  good  features  of  a  book  of 
methods  with  handbook  information  covering  these  methods. 
If  this  book  shall  help  young  repairmen  to  absorb  information 
that  can  be  secured  otherwise  only  through  years  of  experience 
in  handling  one  job  after  another,  and  if  the  older  and  more 
experienced  repairmen  find  it  a  handy  source  of  reference  as 
a  supplement  to  their  own  stock  of  information,  then  the  aim 
of  the  author  will  be  accomplished. 

When  material  has  been  taken  from  the  experiences  of 
engineers  and  their  recommendations  on  repair  methods  as 
published  in  the  technical  journals,  it  has  been  the  aim  to 
give  credit  to  both  the  author  and  the  journal  in  the  para- 
graph, or  section  where  the  material  is  used.  Special  acknowl- 
edgment is  made  to  A.  H.  Mclntire,  editor  of  the  Electric 
Journal,  for  permission  to  make  liberal  use  of  information  con- 
tained in  several  articles  compiled  at  his  suggestion  and  pub- 
lished in  the  Journal.  This  material  has  been  incorporated 
in  Chapters  3,  8,  9  and  11.  To  A.  M.  Dudley,  engineer  of  the 
industrial  division,  Westinghouse  Electric  &  Manufacturing 
Company,  the  author  is  also  especially  grateful  for  sugges- 
tions and  for  permission  to  use  details  of  methods  which  he 
has  developed  for  reconnecting  and  testing  induction  motor 
windings.  This  information  appears  in  Chapters  9  and  11. 
The  diagrams  at  the  end  of  Chapter  11  have  been  selected 
from  a  series  of  eighty-one  devised  by  Mr.  Dudley  and 
shortly  to  be  published  in  a  valuable  treatise  on  "Connecting 
Induction  Motors." 

The  author  also  desires  to  acknowledge  the  assistance  ren- 
dered by  H.  S.  Rich  and  Alex  R.  Knapp  in  the  form  of  data 


PREFACE  Vll 

:•''••'  !•"'•'•.-• 

and  information  compiled  from  their  own  experiences  in  solving 
a  variety  of  motor  troubles  met  with  in  industrial  plants  and 
in  making  repairs.  To  Henry  Scheril,  formerly  a  member  of 
the  engineering  department  of  the  Crocker- Wheeler  Company, 
acknowledgment  is  made  for  helpful  suggestions  in  arranging 
the  material  and  for  assistance  in  checking  and  reading  the 
proof.  Credit  is  also  due  and  is  hereby  accorded  to  the  elec- 
trical manufacturers  who  furnished  the  photographs  from 
which  many  of  the  halftone  illustrations  were  made. 

DANIEL  H.  BRAYMER. 
NEW  YORK  CITY, 
December,  1919. 


INTRODUCTION 

Through  the  courtesy  of  the  author  of  this  book  the  writer 
has  had  the  privilege  of  reading  the  proofs.  I  have  found, 
with  great  delight,  that  the  treatment  of  the  subjects  discussed 
is  not  only  clear  and  easily  understood  but  always  from  the 
practical  man's  standpoint.  While  the  book  will  appeal 
strongly  to  practical  men  engaged  in  repair  shop  work,  power 
station  work  and  the  maintenance  of  motors  in  industrial 
plants,  it  will  also  appeal,  in  the  opinion  of  the  writer,  to 
students  of  electricity.  Since  the  material  presented  in  this 
book,  which  I  dare  say  is  unique  in  its  field,  has  been  ob- 
tained from  actual  practical  experiences  and  outlines  the 
practical  remedies  that  have  been  applied  by  repairmen  in  the 
solving  of  puzzling  problems,  it  will  be  of  decided  assistance 
to  men  who  are  in  need  of  such  practical  help. 

It  is  an  "  electrical  book  of  knowledge,"  for  in  its  pages 
readers  will  find  answers  to  practically  all  armature  winding 
questions  and  solutions  of  many  of  the  repair  problems  that 
they  will  meet  in  practical  work.  The  diagrams  are  clear  and 
easily  followed  by  the  shop  man  and  run  in  synchronism  with 
the  text.  Theory  with  mathematical  considerations  have  been 
resorted  to  only  in  a  very  few  cases  so  that  the  reader  of  the 
book  can  make  use  of  the  information  and  understand  the 
discussions  of  all  phases  of  armature  winding  even  though  he 
may  have  only  a  limited  knowledge  of  mathematics. 

A  book  of  this  kind,  in  spite  of  the  errors  that  are  bound  to 
creep  in,  is  a  very  valuable  asset  to  any  practical  man  who  de- 
sires to  enlarge  his  own  stock  of  knowledge  by  learning  how 
other  men  in  similar  positions  have  solved  the  many  electrical 
problems  that  come  to  the  repairman. 

HENRY  SCHEBIL. 

NEW  YORK  CITY, 

December,  1919. 


IX 


CONTENTS 

PAGE 

PREFACE  ........  f . ,.      v 

INTRODUCTION.    .  \    ....    .....  ix 


CHAPTER  I 


DIRECT-CURRENT  WINDINGS 

Action  of  a  Commutator .'..'.    .  1 

Types  of  D-C  Armature  Windings i    .    .  ^    .    .    .  2 

Winding  Parts  and  Terms 1    ....'..  2 

Armature  Conductor  or  Inductor '...?.    .  3 

Winding  Element  or  Section  .    .-.  V..:;.'H'.    ,  V   .    .  ".    ...  3 

Armature  Coils.    .    ^i~. '..?-.    ,;  i    .    .N  .    .    ; '  . "".  -  .    '.    ,    .    .    .  4 

Winding  Pitch  or  Coil  Pitch  . 6 

Front  and  Back  Pitch.    .  ;vV  v  .    .    ,    :   .    .    .    .    .    .    .    .    .    ;  '%. 

Full  Pitch  and  Fractional  Pitch  Coils  .    .    .   -.    .    .  \   .    .  '.    .    . '^ -  '  t 

Symbols  Used  in  Winding  Formulas     .    .    .    .    .":..,/  .    .    /  8 

Numbering  Coil  Sides  in  Armature  Slots     .    .VV.Y  .    ...  8 

Lap — Multiple  or  Parallel  Windings .    .    .    .    .........  9 

Formulas  for  Lap  Windings — Multiple;  Single,  Double  and 
Triple  Windings — Meaning  of  the  Term  Reentrant — Multi- 
plex Lap  Windings. 

Wave — Series  or  Two-circuit  Windings 16 

Formulas  for  Wave  Windings — Multiplex  Wave  or  Series- 
Parallel  Windings — Formulas  for  Series- Parallel  Windings — 
Symmetrical  Windings. 
Possible  Symmetrical  Windings  for  D-C  Machines  of  Different 

Numbers  of  Poles 21 

Equipotential  Connectors T   .    .'    ;    .    .  :.    .    .    .  21 

Best  D-C  Windings  for  a  Repair  Shop  to  Use    .    .,;;..'..    .  23 

Number  of  Armature  Slots .....';.    .  23 

Voltage  between  Commutator  Segments .    .    ....    ......  24 

Number  of  Commutator  Bars 24 

Usual  Speeds  and  Poles  of  Different  Sizes  of  Generators.    ...  25 

Safe  Armature  Speeds .    .    .    ...  r.;.vv  .......';.    .  25 

CHAPTER  II 

ALTERNATING-CURRENT  WINDINGS 

Types  of  A-C  Windings ,    i    .  27 

Distributed  Windings „    .    .  27 

xi 


xii  CONTENTS 

PAGE 

Concentrated  Windings  .  .7.  .... ..'..' 27. 

Spiral  or  Chain  Windings .    .    -   .    .    „    .    .     27 

Lap  and  Wave  Windings .'.    :    .    .    ;    ...     28 

Whole-coiled  and  Half-coiled  Windings    .    .    *  v  V^'v  *' ^L'V  .    .      30 
Single-phase  and  Polyphase  Windings .......,;....     30 

Coil  Pitch ../I' 32 

Phase  Spread  of  Winding 32 

Two-phase  from  Four-phase  Windings     .    .../.....    .    .    .     33 

Three-phase  from  Six-phase  Windings .    .    .    ...    .......     33 

Wire,  Strap  and  Bar  Wound  Coils 34 

Methods  of  Laying  out  and  Connecting  A-C  Windings  ....  35 
Group  Windings — Full  and  Fractional  Pitch  Windings — 
Simple  Winding  Diagram — Reconnecting  a  Winding — Simple 
Method  for  Indicating  Polarity  of  Coil  Groups — Changing  Star 
to  Delta  Connection — A-C  Wave  Windings — -Progressive  and 
Retrogressive  A-C  Wave  Windings — Connections  for  Coils 
of  Polyphase  Windings — Double-layer  Winding,  Lap  Con- 
nected— Connecting  a  Chain  Winding — Other  Common 
Windings. 

Easily  Remembered  Rules  for  Arrangement  of  Coils  in  an  Induc- 
tion Motor „    .    .;,  , 51 

Simple  Rules  for  Checking  Proper  Phase  Relationship  in  Two- 
or  Three-phase  Windings  .    .   ,. -., ...„. 52 

CHAPTER  III 

REPAIR  SHOP  METHODS  FOR  REWINDING  D-C  ARMATURES 

Dismantling  a  D-C  Armature 56 

Winding  Data  Needed  for  a  Dismantled  Armature 57 

Removing  Old  Coils 57 

Winding  D-C  Armatures  Having  Partially  Closed  Slots  ....     60 
Winding  a  Threaded-in  Coil — Insulating  Lining  for  Slots — 
Inserting  Coils  in  the  Slots — Insulating  Overlapping  End  Con- 
nections of  Coils — Connecting  Finish  Ends  of  Coils  to  Com- 
mutator— Loop  Windings  for  Small  Motors. 

Winding  D-C  Armatures  Having  Open  Slots 66 

Winding  and  Insulating  Coils — Insulating  Open  Slots — In- 
serting Coils  in  Open  Slots — Shaping  End  Connections- 
Truing  up  the  Heads  of  the  Winding — Insulation  Between 
Commutator  End  Connections. 

Winding  Large  D-C  Armatures 77 

Coils  for  Large  D-C  Armatures — Lap  and  Wave  Windings  for 
Large  Armatures — Insulating  the  Core — Inserting  the  Coils — 
Banding  Wire — Balancing  Large  Armatures — Rotary  Con- 
verters— Three-wire  Generators. 


CONTENTS  xiii 

PAGE 

Winding  Railway,  Mill  and  Crane  Types  of  Armatures    ....     90 
Railway  Type  Armature  Coils — Coil  Insulation — Insulating 
the  Core  of  Railway  Armatures — Inserting  the  Coils — Con- 
nections with  Dead  Coils — Hooding  and  Banding. 


CHAPTER  IV 

MAKING  CONNECTIONS  TO  THE  COMMUTATOR 

Locating  First  Connection  to  Commutator 101 

Testing  Out  Coil  Terminals 102 

Commutator  Connections  for  a  Lap  Winding.    .    .    .    .    .    .  '  .    .  102 

Requirements  of  a  Lap  Winding 103 

Commutator  Connections  for  a  Wave  Winding 104 

Locating  First  Connection  to  Commutator  for  a  \Vave  Winding .  105 

Requirements  for  a  Wave  Winding 107 

Progressive  and  Retrogressive  Wave  Windings  .    .    .    .'".    .    .    .  107 

Wave  Winding  with  Dead  Coils    .  V.    : 108 

Cutting  out  Coils  of  a  Retrogressive  and   Progressive  Wave 

Winding 109 

Tables  for  Placing  Coils  and  Connecting  Them  in   a  D-C  Winding  110 

Wave  vs.  Lap  Windings '....,'   l!    .    ..'...    .    .  112 

Lap  Windings  for  D-C  Armatures     .    .    .   \  •''.*/."•  '".    .    .   '.'  .    .    .  115 

Lap  Windings  for  A-C  Machines '.,-.  .    .    .  116 

Wave  Windings  for  D-C  Armatures ,; V/'V  .    .    .  117 

Wave  Windings  for  A-C  Machines ;,v.    .-   .    .    .    .118 

Single  vs.  a  Number  of  Independent  Windings  .    ;    .    .    .    .    .    .118 

Lap  Windings  vs.  Multiple  Wave  Windings 119 

Use  of  Equalizer  Rings   .-.  T  ..../:...'.;:' ^    ....  120 


CHAPTER  V 

TESTING  DIRECT-CURRENT  ARMATURE  WINDINGS 

Causes  of  Short  Circuits  in  an  Armature.    .    .    ...    ...  ..  ,.    .    .    .  122 

Test  for  Short  Circuits  in  an  Armature .    .    .    ......    .  .'    .  v  .    .  123 

Testing  for  Short  Circuits  and  Open  Circuits  with  a  Small  Trans- 
former ...-;.    /v  «    ...   :   ,....,.    .    .  125 

Causes  of  Open  Circuits  in  an  Armature 127 

Tests  for  an  Open  Circuit  in  an  Armature 128 

Cutting  Out  Injured  Coils 129 

Causes  of  Grounds  in  an  Armature 130 

Tests  for  Grounds  in  an  Armature 131 

Use  of  a  Bar  Magnet  and  Millivoltrneter  to  Locate  a  Reversed 

Armature  Coil 

Use  of  a  Compass  to  Locate  a  Reversed  Armature  Coil    .    .    .    .132 

Locating  Low  Resistance  or  Dead  Grounds 133 


xiv  CONTENTS 

PAGE 
Use  of  a  Telephone  Receiver  in  Testing  for  Short  Circuits,  Open 

Circuits  and  Grounds 135 

Testing  for  Reversed  and  Dead  Field  Coils 136 

The  Commutator : ,..    .s  .    .  136 

Testing  Equipment  for  a  Repair  Shop .    .,,.-.    ...    .    .    .    .    .  138 

CHAPTER  VI 

OPERATIONS  BEFORE  AND  AFTER  WINDING  D-C  ARMATURES 

Stripping  Off  an  Old  Winding 139 

Cleaning  and  Filing  Slots 140 

Testing  Commutator -,   ,  y-  ....  140 

Making  New  Coils 140 

Forms  for  Winding  Coils f    .....  141 

Insulation  of  Core  and  Slots 144 

Testing  Out  the  Winding V:*   j    y;v.^    A    .    .    .  144 

Soldering  Coil  Leads  to  the  Commutator    .    ,    .  .,-.,,.    .    .    .    .    .  144 

Hoods  for  Armatures  .    .    ......  ...  .„  .    •    .,.-..    .    .    .    .  145 

Banding  Armatures ,. .    .....    .  146 

Seasoning  and  Grinding  a  Commutator   .    .    ,-  .....    ...    .    .  148 

Undercutting  Mica  of  a  Commutator  .    ....    .  '*. 150 

Balancing  an  Armature  .,  .    ...   „ '...-.-.  ..    ^   .  ...  •    *    •    •- ,  •    •  150 

Painting  the  Winding  .    .    .    .    N.  ..;.*    ...    .,.<..    ...  •    •    •    •    •  151 

Relining  Split  Bearings   .    .    .  ...    ,    .  :.    .    ,    '.    .    ,    .    ....    .  151 

CHAPTER  VII 

INSULATING  COILS  AND  SLOTS  FOR  D-C  AND  A-C  WINDINGS 

Insulation  for  Armature  Coils  and  Slots  .    .    .    .    . 153 

For  Mechanical  Protection  and  Electrical  Insulation — For 
High  Temperatures  and  Electrical  Insulation — For  Electrical 
Insulation  Only. 

Descriptions  and  Uses  of  Insulating  Materials   .....    .    .    .  156 

Treated  Cloths — Pressboards,  Fibres  and  Papers — Coil  and 
Slot  Insulation  Used  in  One  Large  Repair  Shop — Micarta- 
folium. 

Thickness  of  Insulation  Required  in  Slots 163 

Insulation  of  Formed  Coils -.    »  ".    .    .    .    .  163 

Insulation  for  Coils  Used  in  240-Volt  and  500-volt  D-C  Machines.  165 

Coil  Insulation  for  Induction  Motor  Windings  .    .    .    /  ;    .    .    .  167 

Coil  and  Slot  Insulation  Employed  by  a  Large  Manufacturer.    .  168 

Insulation  of  End  Connections  of  Coils    ....   •.  '  /  :r.  •  .'   .    .  172 

Phase  Insulation  When  Reconnecting  from  2-phase  to  3-phase  .  173 

Mica  Insulation  for  Armature  Coils .!•.;-..    .    .  173 

Repairing  Coils  Damaged  in  Winding  Process    .  -.    .    .'"'.    .    .    .  174 

Voltage  to  Use  When  Testing  Coil  and  Commutator  Insulation.  175 


CONTENTS  xv 

PAGE 

Field  Coil  Insulation .  179 

Varnishes  and  Impregnating  Compounds  for  Coils 176 

Characteristics  of  Insulating  Varnishes 178 

Solvent  Chart  for  Insulating  Varnishes 178 

Method  for  Making  Tape  from  Cotton  Cloth .  181 

Drying  Out  Insulation  of  D-C  Generators 181 

Drying  Out  Insulation  of  Synchronous  Motors 182 

Drying  Out  Induction  Motors 183 

Measuring  Insulation  Resistance •>;;'••;>••-.•   .  185 


CHAPTER  VIII 

REPAIR  SHOP  METHODS  FOR  REWINDING  A-C  MACHINES 

Winding  Small  Single-phase  Motors.  .  ...'..  .  i- •;''';".'".''..  .  186 
Insulating  Lining  for  Slots — Winding  the  Skein  Coil — Insert- 
ing Skein  Coil  in  Slots — Winding  for  a  Repulsion-start  Motor 
— Winding  Small  Motors  by  Hand — Windings  for  Odd  Fre- 
quencies— Connections  for  Main  and  Starting  Windings — 
Testing  Small  Induction  Motor  Windings — Windings  for 
Small  Polyphase  Induction  Motors. 

Winding  Small  Induction  Motors  with  Formed  Coils  in  Partially 

Closed  Slots ;  *'..;..   .    .     194 

Insulation  for  Slots — Basket  Coils — Winding  a  3-phase 
Stator  with  Basket  Coils — Threaded  Diamond  Coil — Winding 
a  3-phase  Stator  with  Diamond  Coils. 

Winding  Induction  Motors  Having  Open  Slots.    .    .    .    ;  • .    .    .   201 
Winding  a  2-phase  Stator  Having  Open  Slots — Testing  the 
Windings — Inserting  a  New  Coil  in  a  Winding — Connecting 
the  Coils — Points  to  Consider  when  Connecting  Coils — Cleats 
and  Terminals — Painting  Winding. 

Induction  Motor  Secondaries .   207 

Squirrel-cage  Secondaries — Phase- wound  Secondaries. 

Winding  Large  Alternating  Current  Stators  .  .  . '  .  v  . .  •.  .  .  210 
Coils  for  Partially  Closed  Slots — Coils  for  Open  Slots — Lap 
and  Wave  Connections — Insulation  of  Coils — Inserting 
Shoved  Through  Concentric  Coils — Bar  and  Connector  Wind- 
ing— Diamond  Coils — Double  Windings — Testing  Windings 
of  Large  Machines — Connecting  the  Coils — Bracing  Needed 
for  Heavy  Windings. 

Winding  the  Stator  of  Alternating  Current  Turbo-generators .    .   223 
Coils  for  A-C  Turbo-generators — Forming  the  Coils — Insula- 
tion  for   Turbo-generator    Coils — Testing    Turbo-generator 
Windings — Inserting  the  Coils  in  a  Turbo-generator — Bracing 
for  Windings — Connecting  the  Winding — Break-down  Test. 


xvi  CONTENTS 

CHAPTER  IX 

PAGE 

TESTING  INDUCTION  MOTOR  WINDINGS  FOR  MISTAKES  AND  FAULTS 

Testing  for  Grounds  and  Short-circuits    .    .    .    .    .^  „.,'..    .  231 

Reversal  of  One  or  More  Coils  or  Groups   .    .    .    .    .>  ,    .    .    .    .  233 

Open  Circuits v  .    ..>    .    .    .  234 

Placing  Wrong  Number  of  Coils .,:.,.*..    .N  .    .  234 

Using  an  Improper  Group  Connection .    .    .  234 

Order  in  Which  Tests  Should  be  Made f; -„    .    .    .  235 

Connecting  for  the  Wrong  Number  of  Poles 235 

Applying  Direct  Current  and  Exploring  with  a  Compass.    .    .    .  236 

CHAPTER  X 

ADAPTING  DIRECT-CURRENT  MOTORS  TO  CHANGED  OPERATING  CON- 
DITIONS 
Changes  in  Speed.    .    .    .^  ......  .....r  ,.'...«-.-..    .    .   237 

Changes  in  Operating  Voltage   .    .    .    »fe ,    .>,:.: .    .    ,    .    ."  .   238 
Operating  a  Motor  on  One-half  or  Double  Voltage    .    .    .    ,    .    .   239 
Size  of 'Wire  for  D-C  Armature  Coils   .   >  .«    •    •    •    ..  /  .  ^  .    .    .   240 
Operating  a  Generator  as  a  Motor  and  Vice  Versa    .    .....    .    .   241 

Motor  Speed  when  Reconnecting  a  D-C  Motor  Winding  Wave 

to  Lap 242 

Adjusting  the  Air  Gap  on  Direct-Current  Machines.    ;.  •N.v'.,.    .    .   242 
jChange  in  Brushes  when  Reconnecting  D-C  Motor  from  a  Higher 

to  a  Lower  Voltage 243 

Rewinding  and  Reconnecting  D-C  Armature  Windings  for  a 

Change  in  Voltage 243 

Reconnecting  a  Lap  Winding — Reconnecting  a  Wave  Winding 

— Reconnecting  Duplex  Windings. 

CHAPTER  XI 

PRACTICAL  WAYS  FOR  RECONNECTING  INDUCTION  MOTORS 

Points  to  Consider  before  Making  Reconnections  .......   262 

Diagrams  for  Different  Changes  of  Connections    .  ....   262 

Diagrams  for  Three-phase  Motors 265 

Use  of  Table  of  Connections V?^>;.    .   267 

Two-phase  Diagrams  .    ..  ^  V  , >-...-  v    .-.    .    267 

Meaning  of  the  Term  Chord  Factor.    .    ..  .  ...  v  .-, \ :*  •  .    .    .    .   269 

Phase  Insulation V.  ,    I  •'-.-    .    .    .   271 

Reconnecting  Motors  to  Meet  New  Conditions.    / 272 

Procedure  when  Considering  a  Reconnection  of  Windings — 
Practical  Example  for  Reconnection — Changes  in  Voltage 
only  with  all  Other  Conditions  Remaining  the  Same — 
Changes  of  Phase  Only — Changes  in  Frequency — Changes  in 
Number  of  Poles — Testing  a  Reconnected  Winding. 


CONTENTS  xvii 

PAGE 

Effects  of  High  and  Low  Voltage  on  Motor  Operation 283 

Operating   Standard  A-C   Motors  on   Different  Voltages  and 

Frequencies.    ,,-.... 284 

Factors  which  Limit  a  Change  in  Number  of  Poles  of  an  Induction 

Motor  .    .   ".    ., 286 

Single-circuit  Delta  and  Double-circuit  Star  Connections     .    .    .   287 

Cutting  Out  Coils  of  an  Induction  Motor 288 

Procedure    When    Connecting    Coils   of    an    Induction    Motor 

Winding 288 

Connecting  Pole-phase-groups  of  a  Winding — General 
Theory  on  Which  Connection  Diagrams  are  Constructed — 
Determining  Number  of  Poles  from  Slot  Throw  of  Coils — 
Typical  Circle  Diagrams  for  Connecting  Induction  Motors. 

CHAPTER  XII 
COMMUTATOR  REPAIRS 

Causes  of  Commutator  Troubles ...'.....    .    .  301 

Troubles  Resulting  from  High  Mica 301 

Remedy  for  High  and  Low  Bars .    •    •    •    •    . '"  .    •    •  302 

Burn-out  Between  Bars  .    .    .    .    .    .    .    .    .    .    .....    .    ,    .  303 

Plugging  a  Commutator.    .V.    ;  V':  :.    ..,...;    .    .    .    .    .  304 

Removing  Bars  and  Mica  Segments  for  Repairs    »    .    .    .    .    .    .  304 

Repairing  a  Burned  Commutator  Bar      .    .    ..'..:  ...    .    .  305 

Replacing  a  Repaired  Commutator  Bar  .    .    .    .  -,:  .    .   >.    .    .    .  306 

Tightening  up  a  Repaired  Commutator  .    .    ...    ....    ...  306 

Baking  Commutator  with  Electric  Heat      .   ..    .    »,.  ;;;->   •    •  308 

Removing  and  Repairing  Grounds  in  a  Commutator 308 

Turning  Down  a  Commutator  without  Removing  Armature  .    .  309 

Temporary  Cover  for  Use  When  Turning  Down  a  Commutator.  310 

Refilling  a  Commutator 311 

Boring  Out  the  End  of  a  Commutator ,  \  ^ .    .    .  313 

Mica  Used  in  Commutators ...    •  >   .-    •    -315 

Shaping  Mica  End  Rings .    .  316 

Templet  for  Making  Mica  End  Rings      '.    .    .    .    /:./,.  316 

Micanite  as  a  Commutator  Insulation .    .    .;.  '.    .  317 

Precautions  when  Tightening  a  Commutator.    ... ,  .    ... •'.  . .    •  317 

Making  Micanite  End  Rings ' .    .    . 

Causes  of  Excessive  Commutator  Wear  .    .    .    .    .  ' .    .    .    . 

Copper  Used  for  Making  Commutator  Bars  .....            .    .  319 

Test  for  Oil-saturated  Mica  in  a  Commutator    . 
Blackening  of  a  Commutator  at  Equally  Spaced  Points  . 

Undercutting  Mica  of  Commutators 320 

Tools  for  Undercutting  Mica— Size  of  Circular  Saw  Required 
—Finishing  Slots  and  Commutator  Surface  after  Undercut- 
ting— Brushes  for  Use  on  Undercut .  Commutators — Hand 
Tools  for  Undercutting  Mica  of  Commutators. 


xviii  CONTENTS 

CHAPTER  XIII 

PAGE 

ADJUSTING  BRUSHES  AND  CORRECTING  BRUSH  TROUBLES 

Fitting  or  Grinding-in  Brushes  .    .    .    v 326 

Adjustment  of  Brushholders 329 

Causes  of  Rapid  Brush  Wear ,    .  %    ,    .    .331 

Methods  for  Locating  the  Electrical  Neutral  in  Setting  Brushes .   332 

Angle  at  Which  Brush  is  Set.    .    .   v  „ 333 

Checking  Brush  Setting 334 

Brush  Pressure /. ',    .,!.=  .'.    .    .   334 

Common  Brush  Terms .vvs.    »:,    .    .   334 

Procedure  for  Locating  Causes  of  Brush  Troubles.    .    .    ...    .   337 

Too  Low  Brush  Pressure — Incorrect  Spacing  of    Brushes — 
Brushes   Not   Operating   on   Electrical   Neutral — Incorrect 
Thickness  of  Brushes — Using  Brushes  of  Wrong  Character- 
istics. 
Causes  and  Remedies  for  Sparking  at  Brushes  .    ....    .  ...    .    .   341 

Causes  and  Remedies  for  Flat  Spots  on  Commutator  .....   342 

Causes  and  Remedies  for  Blackening  of  Commutator  .    .    .    .    .  343 

Causes  of  Heating  in  a  Motor  or  Generator    .    .    .-  \    .    ...    .  343 

Causes  and  Remedies  for  Honey-combing  of  Brush  Faces    .    .    .   343 

Causes  and  Remedies  for  Brushes  Picking  up  Copper 343 

Causes  and  Remedies  for  Brushes  Chattering     .... N^   •    •    •   344 
Causes  and  Remedies  for  Loosening  of  Brush  Shunts   ....    .  344 

CHAPTER  XIV 

INSPECTION  AND  REPAIR  OP  MOTOR  STARTERS,  MOTORS  AND  GEN- 
ERATORS. 

Cost  of  Repairs  for  Polyphase  Motors 346 

Points  to  Consider  When  Estimating  Cost  of  Motor  Repairs .    .   347 
Inspection  and  Overhauling  of 

Direct-Current  Motor  Starters.    .  •>.    .    X 349 

Auto-starters  for  A-C  Motors  .    .... 351 

Drum  Type  Controllers .    ...    .    .   %    .    .    .  355 

Large  Compound  D-C  Motor 358 

50-Hp.  Induction  Motor  .    ...    .    .    .    , 363 

25-Hp.  Slip-ring  Motor ,.    .    ...    .    .    .    .    .    ;.*'.   366 

Single-phase  Commutator  Motor .    .    .    .    . :.'.    .    .  '-.    .    .    .    368 
Direct-Current  Engine  Type  Generator.    .    .    .    <    .    ....    .  371 

CHAPTER  XV 

DIAGNOSIS  OF  MOTOR  AND  GENERATOR  TROUBLES 

Lack  of  Proper  Cleaning >..*••    •    •  376 

Operation  in  Damp  Places .    .    .    .  7  .    *^,.  .    ;..,.*.:>    •    •    •  377 

Exposure  to  Acid  Fumes  and  Gases 377 

Lack  of  Frequent  Inspection  and  Replacement  of  Worn  Parts .    .  378 


CONTENTS  xix 

PAGE 

Operating  Temperatures  too  High 378 

Electrical  Defects 379 

Causes  and  Remedies  for  Troubles  in  A-C  Machines 386 

Induction  Motor  Troubles — Locating  Troubles  in  Windings 
of  Induction  Motors — Mechanical  Adjustments — Troubles 
Due  to  Electrical  Faults — Troubles  with  Synchronous  Motors 
— Causes  of  A-C  Motor  Fuses  Blowing — Inspection  of 
Motor  Starting  Devices — Testing  Motor  for  Grounds — Hot 
Stator  Coils — Tension  of  Belts — Troubles  in  Rotor  Wind- 
ings—  Examination  of  Stator  Winding — Sparking  at  Slip 
Rings. 


CHAPTER  XVI 

METHODS  USED  BY  ELECTRICAL   REPAIRMEN  TO   SOLVE  SPECIAL 
TROUBLES 

Sparking  at  Commutator  Caused  by  Poor  Belt  Joints 394 

Plugging  a  Commutator Uv;.'.    ,    .   v  v   .  •.'  *    .    .   395 

Knock  in  an  Armature  Due  to  Band  Wires  Being  too  High .    .   395 
Heating  of  Armature  Traced  to  Poor  Soldering  of  Commutator 

Connections 396 

How  a  Commutator  was  Repaired  under  Difficulties 397 

Holder  for  Sandpapering   Commutator ;    .    .    .   400 

Use  of  Portable  Electric  Drill  to  Undercut  Mica 400 

Jerky  Operation  of  New  Commutator  Traced  to  Burred  Commu- 
tator Bars .    .    .    .    .'-..'  .....  402 

Why  Brush  Studs  Heated  on  an  8-pole  Machine   .    .    .  ..- ,    .    .    .   403 

An  Accident  Due  to  Incorrectly  Set  Brushes 404 

Wrong  Setting  of  Brushes  for  Direction  of  Rotation  Caused  1  lotor 

to  Flash .V.    .-,'.   405 

Proper  Adjustment  of  a  Reaction-type  Brush-holder 406 

Heating  of  Brush-holders  Traced  to  Defective  Contact  Springs .   407 

Simple  Scheme  for  Banding  Armatures 408 

Use  of  a  Crane  to  Band  an  Armature  .    .    .  '.'  .    .    .  ".    .    .    .    .   409 
Method  Used  to  Band  a  2000-Hp.  Rotor    .    '.,  .  V  :    .  ....    .   410 

Improvised  ft  ethod  Used  to  Turn  a  Commutator 411 

Cause  of  a  Motor  Reversing  its  Direction  of  Rotation  on  High 

Speed •  _.;•'•: 

Checking  Connections  of  an  Interpole  Motor 

Heating  of  Field  Coils  Traced  to  Wrong  Type  of  Starting  Box .    .414 

Safe  Operating  Temperature  of  Portable  Desk  Fans 415 

An  Adjustable  Shunt  for  Series  Fields  of  Exciters.    .  .416 

A  Peculiar  High-speed  Motor  Trouble.    .    .    ...'....        .   417 

Ways  that  End  Play  Variations  Show  Up .418 

Connections  for  Two  220-volt  Motors  When  Operated  on  440 
Volts.  .\. 419 


XX  CONTENTS 

PAGE 

Cleaning  Motors  with  Compressed  Air 420 

Testing  out  Phase  Rotation 420 

An  Induction  Motor  Trouble  Due  to  Wrong  Stator  Connections .  421 
Stalling  of  Wound  Rotor  Induction  Motor  Explained  .  ....  422 
Loose  Bearing  Caused  Induction  Motor  to  Fail  to  Start  ....  423 

Three-Phase  Motors  Used  on  Single-phase  Lines 425 

An  Apparent  Overload  Trouble  That  was  Traced  to  a  Defective 

Fuse  Block 427 

Cause  of  Noise  in  Three-phase  Motor  Driving  Exhaust  Fan.  .  428 
Cause  of  Burned  out  Starting  Winding  in  a  Single-phase  Motor.  429 
Cause  of  One  Motor  Failing  to  Start  While  Another  was  Running 

on  Same  Circuit 430 

Cause  of  Synchronous  Motor  Failing  to  Start    .    ....    .    .    .   432 

Effect  of  Decreased  Frequency  on  Induction  Motor- Generator  Set  433 
Simple  Rules  for  Reconnecting  A-C  Motors    .    .    .    .    .    ...    .   434 

Changing  440-volt  Motor  for  Operation  on  220  Volts  .    •  ;  -'    •    •   435 
Multiple  Connection  Diagram  for  A-C  Motor  Windings  ....   436 

Brush  and  Slip-ring  Sparking  Traced  to  Absence  of  Rotor  Balanc- 
ing Weights 437 

Overheating  of  an  Induction  Motor  Traced  to  Variation  in  Fre- 
quency. „    »   .    .    .    .  V  .    .    *   ,  ".    .    .    .    .    .    .    .  V  ,    ..    .   438 

Relief  for  a  Hot  Bearing.    .    .  :.    .    .    .    .    .  ',.  ,, ...    .    .    .    ,    .    .   440 

Static  Sparks  from  Belts .    .    .  •  • .  _.  -.    ...  o  ...   .  V.    ,-  ...    .   440 

Ratings  of  A-C  Generators.    .    »    .    :    .    .    ...*,.*.*.    .    .   440 

Alternating  Current  Motor  Phase  Rotation    .    .    '.',, ;,  ,-  ....   440 

End  Bells  or  Heads .    ....;.....    ,'..,.,...    .    441 

Brushes  and  Brush  Holders    .  ',. ; .;.. .  .    .    .    .,;.,.....    .   441 

The  Rotor ,=  ,>.!..:.  -.'...-.    .    ...  >    s  A   «    ..\    .   441 

The  Stator ,    .    >.•,-,,;.;.    .    441 

Sizes  of  Fuses  for  A-C  Motors  ......    .";..-..    .    .    .   442 

CHAPTER  XVII 

MACHINE  EQUIPMENT  AND  TOOLS  NEEDED  IN  A  REPAIR  SHOP 

Armature  Winder's  Tools 445 

Device  for  Shaping  Insulating  Cells  of  Armature  Slots 447 

Tool  for  Cutting  Cell  Lining  at  Top  of  Slot    .  : .    . .  .    ...,.'.    .  447 

Special  Winding  Tools 449 

Repair  Tools  that  can  be  Made  from  Old  Hack-saw  Blades    .    .  451 

Special  Coil- winding  Device  .    .    .    .        .    .    .   ,,     rt^->    .        .  452 

Steadying  Brace  for  Repairing  Small  Motors.    .    ,    .•  ...    .    .  454 

Tension  Block  for  Use  when  Banding  Armatures  ...        .    .  454 

Armature  Sling .    .    •  ,      .    .    .    .  „.  V    .    .  455 

Pinion  Puller ...'...  .456 

Coil  Winding  Machines !    .    .   ..    .    ...    .    .    .    .    •  456 

Qoil- taping  Machines  .    ....,..,.,,,;,*.>'<•  4^7 


CONTENTS  xxi 

r  (•  k 

PAGE 
Commutator-slotting  and  Grinding  Machines    .    ; .!. '•-.    .    .    .    .   459 

Armature  Banding  Machine  .  V  .    .    .     '    .    .-.  .  v   .    .    .    .    .    .   462 

Combination  Machines   ......   .; '.-.',    .    .   462 

APPENDIX 

DATA  AND  REFERENCE  TABLES 

How  to  Remember  the  Wire  Table  .    .    .  - .  -  -  \'',    .    .    ,-.    .    .    .  465 

Copper  for  Various  Systems  of  Distribution   .    .    .    .  V  ....    .  465 

Classification  of  Wire  Gauges >    1, .    ....  466 

General  Wiring  Formulas  for  A-C  and  D-C  Circuits .    .    .    .    .  '  468 

Data  for  Connecting  Motors  to  Supply  Circuits    ....    .    .    .  473 

Voltage,  Horsepower  and  Speeds  of  Motors    .    .    >   ^-    •    ,    .    .  478 

Transformer  Rating  for  A-C  Motors    ...        ........  478 

Sizes  of  Fuses.  Switches  and  Lead  Wires  for  Motors  of  Different 

Sizes  on  Different  Voltages .„..-..    .    .    .  484 

Circuit  Breakers  for  Overload  Protection  of  Motors 489 

Belting .''.....    .    .    .490 

Rules  for  Pulley  Sizes .  v    .    .    .    .    .  492 

Speed  of  Pulleys £    ...    ,    .    .v  .    .    .    ...  492 

Chain  Drives _ .    .  493 

Points  to  Consider  when  Calculating  Size  of  Chain 493 

Horsepower  Transmitted  by  Steel  Shafting     .  .A.v 495 

Horsepower  Transmitted  by  Single  Ropes  .  -.    .    ;    .  •  .    .    .    .    .  495 

Gear  Table .'..-. 496 

Some  Handy  Rules  .    .    .   ,    ,    .%    .    /.•'..;.... 498 

INDEX  .                                                                                                    .  500 


ARMATURE  WINDING  S 

AND 

MOTOR  REPAIR 

CHAPTER  I 
DIRECT-CURRENT  WINDINGS 

The  essential  physical  differences  between  a  complete  direct- 
current  and  a  complete  alternating-current  armature  winding 
is  that  the  former  is  wound  on  the  rotating  member  of  the 
machine  while  the  latter  is  wound  on  the  stationary  member 
and  that  the  direct-current  winding  requires  a  commutator 
while  the  alternating-current  winding  does  not.  However, 
since  the  practical  make-up  and  construction  of  windings 
will  be  discussed  later  for  particular  types  of  direct  and 
alternating-current  machines,  the  general  theory  of  arma- 
ture windings  will  likewise  be  taken  up  first  for  direct-current 
and  then  for  alternating-current  machines  (see  Chapter  II). 

Action  of  a  Commutator. — The  emf  and  current  produced 
in  each  armature  conductor  of  a  direct-current  generator  is 
alternating  in  character.  It  is  the  function  of  the  commutator 
to  deliver  from  the  armature  winding  an  electromotive  force 
and  current  that  is  unidirectional,  that  is,  such  that  one  termi- 
nal will  be  always  of  positive  polarity  and  the  other  of  nega- 
tive polarity.  The  commutator  and  its  brushes  accomplish 
this  by  being  connected  in  series  between  the  generator  leads 
and  the  armature  windings  so  as  to  reverse  (in  effect)  the  con- 
n3ctions  of  the  armature  coils  (connected  to  the  commutator 
bars)  with  respect  to  the  machine  leads  every  time  the  emf 
and  current  induced  in  these  coils  reverse  upon  moving  out  of 
the  influence  of  one  pole  into  the  field  of  the  next  adjacent 

1 


2  ARMATURE  WINDING  AND  MOTOR  REPAIR 

pole.  The  alternating  emf  and  current  generated  in  the 
armature  winding  is  thus  rectified  or  commutated  into  a  uni- 
directional emf  and  current. 

In  the  case  of  an  alternating-current  generator  no  such 
rectifying  of  induced  emf  and  current  is  necessary  so  that  the 
coils  or  elements  making  up  the  armature  winding  can  be 
connected  directly  together  with  the  resulting  terminals  of 
the  winding  becoming  the  terminals  of  the  machine. 

Types  of  D.-C.  Armature  Windings. — In  general,  armature 
windings  are  either  of  the  open  circuit  or  the  closed  circuit 
type.  The  latter  is  used  in  all  modern  direct-current  ma- 
chines, while  alternating-current  machines  may  have  either 
open  or  closed  windings.  In  the  closed  circuit  winding  of 
the  direct-current  machine  the  end  joins  up  with  the  begin- 
ning or  re-enters  itself  with  the  commutator  tapped  to  the  wind- 
ing at  equally  distant  points.  In  the  case  of  open  circuit 
winding  of  an  alternating-current  generator  wound  on  the 
revolving  member,  the  ends  terminate  in  collector  rings  and 
the  winding  is  thus  open  until  closed  by  the  brushes  of  the 
external  circuit.  When  the  winding  is  on  the  stator  of  an 
alternating-current  machine  the  ends  are  joined  through  the 
load  circuit.  The  following  classification  of  closed  circuit  or 
direct-current  windings  may  be  made: 

Direct-current  windings  (closed  circuit). 

1.  Lap — multiple  or  parallel. 

(a)  Single  lap. 
(6)  Multiplex  lap. 

2.  Wave — series  or  two-circuit. 

(a)  Single  wave. 

(6)  Multiplex  wave  or  series-parallel  windings. 

Winding  Parts  and  Terms. — In  formulas  for  armature 
windings  and  in  laying  out  a  repair  job,  certain  terms  are  used 
which  refer  to  parts  of  the  armature  winding,  the  armature 
core  and  details  of  the  arrangement  of  the  former  in  the  slots 
on  the  surface  of  the  latter.  In  what  follows  these  terms  are 
explained.  In  most  cases  they  are  used  alike  both  in  windings 
for  direct-current  and  for  alternating-current  machines. 


DIRECT-CURRENT  WINDINGS  3 

Armature  Conductor  or  Inductor. — That  part  of  a  wire 
which  lies  in  an  armature  slot  and  cuts  the  magnetic  lines  of 
force  or  field  flux  as  the  armature  rotates,  is  called  an  armature 
conductor  or  an  inductor. 


FIG.  1. — Different  types  of  coils  used  in  armature  windings. 
(A)  One-piece  series  diamond  strap  coil.  Leads  at  end  of  straight  part.  (B)  One-piece 
series  diamond  coil.  Leads  at  end  of  straight  part.  (C)  Two-piece  series  diamond  coil. 
(JD)  One-piece  multiple  diamond  coil.  Leads  at  point  of  diamond.  (E)  Two-piece 
multiple  diamond  coil.  (F)  Concentric  coil  bent  down  at  both  ends.  (G)  Concentric 
coil,  straight.  (HI  One-piece  wire  wound  involute  coil.  Leads  at  point  of  involute. 
/)  Two-piece  involute  coil.  Leads  at  point  of  involute.  (J)  Threaded-in  type  diamond 
coil.  Leads  at  point  of  diamond  before  and  after  pulling.  (K)  Basket  coil.  (LI  Same 
as  B  of  threaded-in  type.  (M)  Same  as  D  of  threaded-in  type.  (AO  Bar  and  involute 
end  connector.  (0)  Group  of  concentric  end  connectors.  (P)  Concentric  shoved 
through  type  coil  bent  down  on  one  end. 

Winding  Element  or  Section. — That  part  of  an  armature 
winding  which  is  connected  between  two  commutator  bars  is 


4  ARMATURE  WINDING  AND  MOTOR  REPAIR 

called  a  winding  element.  In  its  simplest  form  a  winding 
element  consists  of  a  coil  of  one  turn  of  wire  or  two  conduc- 
tors. An  element  therefore  must  have  at  least  two  conduc- 
tors but  may  consist  of  more  than  one  turn  of  wire  or  even 
number  of  conductors. 

Armature  Coil. — When  a  winding  element  consists  of  more 
than  one  turn  of  wire  or  two  conductors,  it  is  usually  known 
as  a  coil  and  the  winding  is  a  coil  winding  as  distinguished 
from  a  bar  winding,  where  the  conductors  in  the  armature 
slots  are  copper  bars. 

From  a  mechanical  standpoint  an  armature  winding  con- 
sists of  a  number  of  coils  connected  to  a  commutator  in  the 
case  of  a  direct-current  machine  or  connected  together  in  the 
case  of  an  alternating-current  machine  to  form  a  series  or 
group.  Each  coil  may  be  made  up  of  one  turn  of  wire  with 
each  side  forming  one  armature  conductor  or  inductor,  or  a 
coil  may  be  made  up  of  several  turns  of  wire  or  of  copper 
strips.  A  classification  of  the  different  types  and  uses  of 
armature  coils  which  has  been  made  by  R.  A.  Smart 
(Electric  Journal,  Vol.  VII,  No.  6)  is  given  in  the  accom- 
panying table. 

The  so-called  form-wound,  diamond  coils  mentioned  in  the 
table  are  formed  and  completely  insulated  before  being  as- 
sembled on  the  armature.  They  can  only  be  used  in  open 
slots  Concerning  the  advantages  of  diamond  coils,  involute 
coils  and  concentric  coils,  Mr.  Smart  has  the  following  to 
say:  "The  great  advantage  of  diamond  coils  is  the  easy  and 
simple  manner  in  which  they  can  be  manufactured,  especially 
in  large  quantities,  which  makes  them  well  adapted  for  stand- 
ard machines.  Since  all  the  coils  used  on  one  machine  are 
of  the  same  size  and  shape,  only  one  winding  mould  over 
which  to  form  them  is  necessary.  Moreover,  the  number  of 
spare  parts  which  must  be  kept  on  hand  for  repairing  is  re- 
duced and  repairs  can  be  made  easily  and  quickly.  From 
the  electrical  point  of  view,  the  diamond  type  of  winding 
possesses  the  advantage  of  being  absolutely  symmetrical. 
Hence  there  is  no  tendency  for  unbalancing  of  voltages  due 
to  differences  of  self-induction;  and  in  closed  windings  there 
is  no  tendency  to  produce  internal  circulating  currents." 


DIRECT-CURRENT  WINDINGS 


CLASSIFICATION  OF  ARMATURE  COILS  ACCORDING  TO  SLOTS  IN  WHICH 

THEY  ARE  USED,  THEIR  FORM  AND  TRADE  NAMES  AS  EMPLOYED 

IN  BOTH  DIRECT-  AND  ALTERNATING-CURRENT-WINDINGS 


(  Leads  at  ends  of  straight  part 

1  Leads  at  point  of  diamond 

Involute 

j  Leads,  at  ends  of  straight  part 

Open  slots 

\  Leads  at  point  of  involute     ' 

Short  type 

involute 

Leads  at  point  of  involute 

Concentric 

Straight 

Bent  at  both  ends 

Mould  wound 
coil  of  insulated 

Shoved  through  con- 

Straight 
Bent  at  one  end 

wire  or  ribbon 

centric 

Bent  at  both  ends 

Partially 

,' 
Threaded 

„  .       '     ,  J  Leads  at  end  of  straight  part 
.Diamond  <                          . 
|^  Leads  at  point  of  diamond 

closed  slots 

Shuttle 

Leads  at  ends  of  straight  part 

Basket 

Leads  at  point  of  diamond 

Hand-wound,    pulled  j  StraiSht 

,,          ,                           i  Bent  at  one  end 
through  concentric        ._              .      .         . 
1  Bent  at  both  ends 

Diamond                         < 

Open  slots    < 

Involute 

Form  wound 

coil  of  bare 

Concentric 

wire  or  strap 

Shoved  through  con- 

Partially 

centric. 

closed  slots 

Threaded,  diamond      < 

Leads  at  end  of  straight  part 

Leads  at  point  of  diamond 

Leads  at  end  of  straight  part 

Leads  at  point  of  involute. 

Straight 

Bent  at  both  ends. 

Straight 

Bent  at  one  end 

Leads  at  ends  of  straight  part 

Leads  at  point  of  diamond 


Bars  and  con- 
nectors of 
bare  strap 


Partially 
closed  slots 


Involute  end  connectors 
Concentric  end  connectors 


Involute  Coils. — "  Involute  coils  share  the  advantages  of 
the  diamond  coils  in  that  all  are  of  a  standard  size  and  shape. 
They  also  require  less  space  for  end  connection  than  any  other 
form  of  coil.  They  are,  however,  difficult  to  insulate  properly 
on  account  of  the  larger  number  of  bends  and  are  difficult  to 
assemble  in  position  in  the  armature.  For  this  reason  their 
use  is  restricted.  The  bar  type  of  coil  with  involute  end 
connectors  is  easy  to  insulate  and  assemble  and  can  be  readily 
repaired.  Their  principal  use  is  for  direct-current  and  indus- 
trial motors  where  end  space  must  be  reduced  to  a  minimum.  " 

Concentric  Coils. — "Concentric  coils  can  be  used  on  any 
kind  of  slot.  They  can  be  hand-wound,  machine-wound,  or 
'shoved  through'  (a  combination  of  the  other  two  methods), 
as  best  suited.  The  shape  of  coils  is  simple,  hence  they  are 


6  ARMATURE  WINDING  AND  MOTOR  REPAIR 

easy  to  wind  either  on  a  mould  or  by  hand.  They  can  be  ade- 
quately insulated,  and  can  be  securely  braced  with  simple  and 
reliable  coil  support.  However,  the  coils  belonging  to  the 
same  group  are  of  different  size,  and  the  coils  in  different 
groups,  except  on  single-phase  machines,  are  bent  in  at  least 
two  and  often  in  three  different  shapes.  This  is  a  disadvantage 
from  the  electrical  point  of  view,  since  there  will  always  be  a 
tendency  toward  unbalancing  due  to  differences  in  self- 
induction,  and  toward  the  production  of  circulating  currents 
in  closed  windings.  And  it  is  also  a  disadvantage  from  the 
mechanical  point  of  view,  since  for  one  machine,  a  large 
number  of  different  moulds  will  be  required  and  coils  cannot 
be  interchanged.  Hence,  the  number  of  spare  parts  necessary 
for  repairs  is  greatly  increased,  and  both  the  manufacturing 
and  repairing  of  the  winding  will  require  more  time  and  be 
more  expensive." 

Winding  Pitch  or  Coil  Pitch. — The  distance  between  the 
beginning  of  one  winding  element  or  coil  side  to  the  beginning 


Commutator  Bars 


CommutatprJJars 


FIGS.  2  and  3. — Comparison  of  wave  and  lap  windings. 

At  the  left  a  wave  winding  showing  front  pitch  of  coils  as  yi;  back  pitch  as  j/a;  total 
pitch  as  y  and  commutator  pitch  as  yk.  At  right,  a  lap  winding  with  the  same  notations 
for  the  different  pitches  of  coils. 

of  the  next  element  or  coil  side  connected  to  the  first  one  is 
called  the  total  winding  pitch.  This  is  shown  as  y  for  both  lap 
and  wave  windings  in  Figs.  2  and  3.  Winding  pitch  is  meas- 
ured in  number  of  slots  or  by  number  of  elements  or  coil 
sides  spanned  by  a  single  coil  or  by  the  number  of  commutator 
bars  between  the  connections  to  the  commutator  of  the  two 
sides  of  a  coil.  In  the  third  instance  the  measurement  is 
known  as  commutator  pitch  and  indicated  as  yk  in  Fig.  2 


DIRECT-CURRENT  WINDINGS 


and  Fig.  3.  In  this  chapter  unless  otherwise  stated  coil 
pitch  will  be  designated  by  the  number  of  winding  spaces 
or  coil  sides  spanned.  There  are  two  winding  spaces  per 
slot  in  a  double  layer  winding. 

Front  Pitch. — The  distance  between  the  two  coil  sides  con- 
nected to  the  same  commutator  bar,  measured  in  coil  sides 
at  the  front  or  commutator  end  of  the  armature,  is  called  the 
front  pitch.  It  is  indicated  as  y\  in  Figs.  2  and  3. 

Back  Pitch. — The  distance  between  two  sides  of  a  coil, 
measured  in  coil  sides,  at  the  back  end  of  the  armature  is 
known  as  the  back  pitch.  It  is  shown  as  y2  in  Figs.  2  and 
3.  The  total  winding  pitch  y  is  equal  to  the  algebraic  sum 


I  I 


TT 


Fro.  4. — Lap  and  wave  windings  showing  connections  of  formed  coils  to  the 
commutator  in  double  layer  windings. 

of  the  front  and  back  pitches.  That  is,  since  in  a  lap  winding 
the  front  and  back  pitches  are  of  opposite  sign,  being  laid 
off  in  opposite  directions  on  the  armature,  the  total  pitch  y 
will  be  the  difference  between  them.  In  a  wave  winding  where 
the  front  and  back  pitches  are  laid  off  in  the  same  direction,  the 
total  pitch  y  will  be  their  sum. 

Full  Pitch  and  Fractional  Pitch  Coils. — When  a  coil  spans 
exactly  the  distance  between  the  centers  of  adjacent  field  poles 
(known  as  pole-pitch)  it  is  said  to  be  a  full-pitch  coil.  In 
cases  where  a  coil  is  less  than  full  pitch,  it  is  said  to  have  a 
fractional  pitch  or  to  be  a  short-pitch  coil.  Such  a  winding  is 
often  referred  to  as  a  short- chord  winding. 

Fractional  pitch  windings  are  much  used  because  of  the 
following  advantages:  (1)  They  make  possible  shorter  end- 
connections  and  therefore  call  for  less  copper.  (2)  Armature 


8 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


reaction  is  reduced  since  the  currents  in  the  neutral  zone  flow 
partly  in  opposite  directions  and  neutralize  each  other.  (3) 
In  alternating  current  generators  a  smoothing  out  of  the  wave 
of  induced  emf  is  produced  so  as  to  more  nearly  approach  a 
sine  form. 

Symbols  Used  in  Winding  Formulas. — The  symbols  which 
are  much  used  in  winding  formulas  for  the  parts  of  windings 
defined  in  the  preceding  paragraphs  are  as  follows: 

Z  =  Total  number  of  armature  conductors,  (2gC)  or  (2gK). 

C  —  Number  of  winding  elements  or  coils  usually  equal  to  K. 

g    =  Number  of  turns  per  winding  element  or  coil. 

2g  =  Number  of  conductors  per  winding  element  or  coil. 

N  —  Number  of  coil  sides  of  winding. 

y    =  Winding  pitch  or  coil  pitch. 

2/i  =  Front  pitch  of  winding  element  or  coil. 

2/2  =  Back  pitch  of  winding  element  or  coil. 

2/fc  =  Commutator  pitch. 

K  =  Number  of  commutator  bars. 

p    =  Pairs  of  field  magnet  poles. 

2p  =  Number  of  field  magnet  poles. 

2a  =  Number  of  sections  of  armature  winding  in  parallel. 

s    =  Number  of  slots  on  the  armature. 

Numbering  of  Coil  Sides  in  Armature  Slots. — Form  wound 
coils  are  usually  arranged  in  two  layers  with  one  side  of  each 


D 


5|D 
6|  D 


FIG.  5. — Winding  with  two 
coil  sides  per  slot. 


FIG.  6. — Winding  with  four  coil  sides 
per  slot.  The  coil  pitch  in  this  case  is 
7  winding  spaces  or  coil  sides. 


coil  placed  in  the  bottom  of  a  slot  and  the  other  side  in  the 
top.  For  convenience  in  laying  out  windings,  the  coil  sides 
forming  the  top  layers  in  the  slot  are  given  odd  numbers  and 
those  forming  the  bottom  layers  even  numbers.  This  scheme 
of  numbering  is  shown  in  Figs.  5  and  6.  In  those  cases 
where  the  winding  element  consists  of  a  coil  of  a  number  of 
turns  of  wire  instead  of  two  conductors  or  bars,  numbers  are 


DIRECT-CURRENT  WINDINGS  9 

assigned  to  the  consecutive  bundle  of  conductors  or  wires  of 
which  the  coil  is  made  instead  of  assigning  numbers -to  the 
individual  conductors  or  inductors  as  they  are  sometimes 
called.  When  such  coils  are  used,  the  front  and  back  pitches 
are  determined  by  counting  consecutive  half  coils  between  the 
the  two  coil  sides  of  any  coil  beginning  from  one  coil  side  and 
counting  forward  as  the  coil  is  laid  off  and  considering  the 
first  side  of  the  coil  as  one.  That  is,  in  case  of  a  double  layer 
lap  winding,  with  one  coil  side  in  the  bottom  of  slot  3,  and 
the  other  side  in  the  top  of  slot  7,  the  back  pitch  would  be  7 
counted  as  follows,  there  being  two  coil  sides  in  each  slot: 
First  coil  side  6,  next  coil  sides  spanned  are  7,  8,  9,  10,  11  and 
12.  The  number  of  slots  spanned  by  the  coil  is  called  the 
slot  pitch  or  throw  of  the  coil.  The  throw  of  the  coil,  more 
often  called  coil  throw,  in  the  case  mentioned,  where  one  side 
of  a  coil  is  in  slot  3  and  the  other  in  slot  7,  is  4  slots.  In  the 
case  of  a  four-pole  machine  and  an  armature  having  16  slots, 
this  coil  throw  is  a  full  pole  pitch,  that  is,  from  the  center  of 
one  field  pole  to  the  center  of  the  next,  and  the  winding  is  a 
full  pitch  winding. 

LAP— MULTIPLE  OR  PARALLEL  WINDINGS 

-i 
->• 

A  lap  winding  is  so  called  from  the  lapping  back  of  the  coils 
or  winding  elements  as  they  are  wound  on  the  armature  core. 
This  is  shown  in  Figs.  7  and  8.  A  better  name  is  a  multi- 


FIG.  7. — Lap  winding  showing  the  relative  position  of  one  winding  element 
or  coil  of  two  turns  on  the  armature  of  a  4-pole  machine. 

pie  or  parallel  winding  because  of  the  fact  that  such  a  winding 
consists  of  as  many  circuits  as  there  are  field  poles  and  these 
circuits  are  connected  in  parallel  between  the  brushes.  The 
number  of  brushes  required  therefore  equals  the  number  of 


10 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


armature  sections  in  parallel  and  also  equals  the  number  of 
poles.     That  is  a  =  p.     A  lap  winding  can  be  distinguished 


--fh 


9 


1 


6 


FIG.  8. — Winding  diagram  for  the  lap  winding  of  Fig.  7  showing  how 
the  finish  end  of  one  winding  element  or  coil  of  two  turns  is  joined  to 
the  commutator  and  to  the  start  end  of  the  next  coil  under  the  same  pair  of 
poles. 


FIG.  9. — (a)  Lap  winding  showing  both  end  connections  of  each  coil  side 
bent  toward  center  of  the  coil.  (6)  Wave  winding  showing  end  connections 
of  each  coil  side  bent  in  opposite  directions. 

from  a  so-called  wave  winding  from  the  appearance  of  the 
end  connections  of  coils.     In  the  wave  winding  the  front  and 


DIRECT-CURRENT  WINDINGS  11 

rear  ends  of  the  coils  lead  in  opposite  directions  while  in  the 
lap  winding  they  continue  in  the  same  direction  around  the 
armature,  as  shown  in  Fig.  9. 

Formulas  for  Lap  Winding. — The  following  rules  and  restric- 
tions govern  the  assembling  and  use  of  a  lap  winding: 

1.  The  front  and  back  pitches  (in  winding  spaces  or  coil  sides) 
must  both  be  odd  numbers  and  differ  by  two  or  some  multiple 
thereof. 

2.  The  front  and  back  pitches  are  of  opposite  sign  (one  positive 
and  the  other  negative)  since  they  are  laid  off  in  opposite  direc- 
tions on  the  armature. 

3.  The  winding  pitch  is  equal  to  the  algebraic  sum  of  the  front  and 
back  pitches.     That  is,  in  case  the  front  pitch  is  +  9  and  the 
back  pitch  —  7,  the  algebraic  sum  is  (9  —  7)  or  2. 

4.  The  total  number  of  armature  conductors  or  inductors  must  be 
a  multiple  of  the  number  of  slots  on  the  armature. 

5.  The  number  of  armature  slots  may  be  odd  or  even. 

6.  The  number  of  current  collecting  points  or  brushes  on  the  com- 
mutator must  equal  the  number  of  poles. 

7.  The  maximum  emf  between  two  consecutive  coil  sides  (top 
and  bottom)  in  the  same  slot,  is  equal  to  or  a  little  less  than 
the  terminal  voltage. 

8.  The  end  of  one  coil  is  joined  to  the  commutator  and  to  the 
start  of  another  coil  (usually  the  next)  under  the  same  pair  of 
poles. 


The  following  formulas  apply  in  lap  windings : 

N  ±  b 
2p 

N  ±b 


Front  pitch       =  y\  =  — ^—  -  ±2 

zp 


Back  pitch         =  yk 

Winding  pitch  =  y  =  algebraic  sum  of  yi  and  y%  =  ±2 
Commutator  pitch  =  yk  =  ±1 

In  the  formulas  for  front  and  back  pitches,  N  (as  given  on 
page  8)  is  the  number  of  coil  sides  in  the  winding;  p  the 
number  of  pairs  of  poles  and  b  any  even  number  which  will 
make  both  t/i  and  y2  odd  whole  numbers  and  about  equal  to 
the  pole  pitch.  The  value  of  b  should  be  taken  as  large  as 
allowable  but  not  too  large  for  then  there  is  liability  of  the 
two  sides  of  a  coil  coming  under  the  influence  of  poles  of  the 
same  polarity  so  that  the  induced  emfs  would  oppose  each 


12 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


other.  For  6  =  0,  the  back  pitch  y2  becomes  equal  to  the  pole 
pitch.  When  b  is  positive  t/2  becomes  greater  than  the 
pole  pitch,  while  with  b  negative,  1/2  becomes  less  than 
the  pole  pitch.  In  most  cases  b  is  taken  negative  so  that  2/2 
is  equal  to  or  less  than  the  pole  pitch. 


FIG.  10.  —  Lap  winding  for  a  4-pole  machine  showing  the  direction  of  current 
flow  in  one  of  the  four  current  paths  for  the  brush  positions  as  illustrated. 

The  current   (i)  which  flows  in  each  conductor  of  a  lap 
winding  is, 


Where  7  is  the  total  armature  current  or  that  flowing  in  the 
external  circuit.  The  value  of  (i)  should  be  considered  when 
selecting  the  proper  size  of  wire  in  making  up  or  ordering  coils. 

The  only  practical  lap  winding  from  the  standpoint  of 
symmetry  is  the  one  in  which  the  number  of  pairs  of  circuits 
(current  paths)  in  parallel  equals  the  number  of  pairs  of  poles 
or  a  =  p. 

A  lap  winding  is  symmetrical  when  s  -f-  a  and  s  -r-  p  is  a 


DIRECT-CURRENT  WINDINGS  13 

whole  number.  Here  s  equals  the  number  of  slots,  a  the 
pairs  of  circuits  in  parallel  and  p,  the  pairs  of  poles.  With 
four-pole  motors  it  is  not  always  found  that  s  -f-  a  and  s  -f-  p 
are  whole  numbers,  since  manufacturers  sometimes  make  the 
number  of  commutator  bars  and  slots  odd  so  as  to  make  the 
armature  suitable  for  a  two-circuit  wave  winding  as  well  as  a 
lap  winding. 

In  lap  windings  where  the  commutator  pitch  (yk)  equals 
one,  when  a  =  p,  the  common  factor  of  the  number  of  commu- 
tator bars,  and  yk  will  always  be  unity  and  there  will  be  a  single 
winding. 

In  a  double  layer  winding  with  two  coil  sides  per  slot,  the 
number  of  slots  equals  the  number  .of  coils  in  the  winding. 
When  there  are  more  than  two  coil  sides  in  a  slot,  the  number 
of  coils,  C  =  sN8  -T-  2  with  no  idle  coils;  where  s  equals  the 
number  of  slots  and  N8  equals  the  number  of  coil  sides  per  slot. 
More  than  two  coil  sides  per  slot  greatly  reduces  the  number 
of  slots  required.  Such  a  winding  is  symmetrical  only  when 
s  -r-  a  is  a  whole  number. 

For  a  lap  winding  the  potential  pitch,  yp  =  K  -~  p,  where  K 
equals  the  number  of  commutator  bars  and  p  is  the  number 
of  pairs  of  poles. 

When  equalized  rings  are  needed  with  lap  winding,  it  is 
good  practice  to  use  one  ring  for  every  6  or  12  commutator 
bars. 

Multiplex,  Single,  Double  and  Triple  Windings. — In  cases 
where  it  is  necessary  that  the  armature  shall  carry  a  very 
heavy  current  more  than  one  winding  may  be  used  on  the 
armature  with  an  equal  number  of  commutator  bars  for  each. 
Both  lap  and  wave  windings  may  be  so  made  up  with  one, 
two  or  three  entirely  separate  windings.  In  the  case  of  a 
double  lap  winding,  the  sides  of  a  winding  element  or  coil 
of  one  winding  are  sandwiched  between  the  coil  sides  of  the 
other  and  likewise  the  commutator  bars  of  ona  winding  are 
sandwiched  between  those  of  the  other.  Each  brush  must 
be  thick  enough  to  always  touch  two  commutator  bars  so  that 
both  windings  will  always  be  connected  to  the  brushes  and  both 
deliver  or  receive  current  evenly.  Three  windings  so  sand- 
wiched make  up  a  triple  winding. 


14  ARMATURE  WINDING  AND  MOTOR  REPAIR 

A  single  lap  winding  always  has  the  same  number  of  current 
paths  between  brush  sets  as  there  are  field  poles  while  a  double 
lap  winding  has  twice  the  number  of  current  paths  as  there  are 
field  poles.  The  triple  lap  winding  has  three  times  the  number 
of  current  paths  as  there  are  field  poles.  A  single  wave  wind- 
ing always  has  two  current  paths  between  brush  sets  while  the 
double  wave  winding  has  four  current  paths  between  brush 
sets  and  the  triple  wave  winding  has  six  such  paths  between 
brush  sets. 

An  armature  has  one  winding  (single)  when  the  number  of 
commutator  bars  K  and  commutator  pitch  yk  have  no  common 
factor;  it  is  double  when  their  common  factor  is  two,  and  it 
is  triple  when  their  common  factor  is  three. 

Meaning  of  the  Term  Reentrant. — A  winding  is  often  said 
to  be  single  or  double  reentrant.  In  the  case  of  a  single 
winding,  this  means  that  the  winding  closes  on  itself  or  re- 
turns to  the  beginning  point  after  being  traced  through  all  the 
coils  upon  passing  once  around  the  armature  core.  A  wind- 
ing is  doubly  reentrant  if  it  only  re-enters  itself  after  making 
two  passages  around  the  coils  of  the  armature.  A  single 
winding  may  be  either  single  reentrant  or  doubly  reentrant. 

In  the  case  of  double  and  triple  windings,  the  term  reentrant 
is  sometimes  used  in  an  improper  sense.  For  this  reason  it  is 
advisable  to  specify  types  of  armature  windings  by  the  num- 
ber of  separate  windings,  used.  Thus  a  single  winding  as  a 
single-closed;  two  windings  as  double-closed  and  three  wind- 
ings as  triple-closed  etc.  r  A  winding  made  up  of  two  single 
windings,  each  of  which  re-enters  itself,  will  therefore  be  a 
double-closed  winding  not  a  "double-reentrant"  one. 

Multiplex  Lap  Windings. — As  explained  under  the  heading 
of  "Multiplex  Windings,"  a  lap  winding  may  be  made  up  of 
one,  two  or  three  separate  windings  in  order  to  handle  heavy 
armature  currents.  In  the  case  of  a  double  lap  winding,  two 
similar  windings  insulated  from  each  other  are  placed  in  the 
armature  slots  with  the  even  numbered  commutator  bars 
connected  to  one  winding  and  the  odd  numbered  bars  con- 
nected to  the  other  winding.  In  the  same  way  for  a  triple 
lap  winding  one-third  of  the  commutator  bars  provided  would 
be  connected  to  each  winding. 


DIRECT-CURRENT  WINDINGS  15 

The  formulas  which  apply  in  multiplex  lap  windings  are  as 
follows : 

Front  pitch  =  yl  =  — ^= —  ±  2m 
zp 

N±b 

Back  pitch  =  y2  =  — 

Zp 

Winding  pitch  =  y  =  y2  —  y±  =  ±  2m 
Commutator  pitch  =  yk  =  -2  =  ±  m 

In  these  formulas  w  is  2  for  a  double  winding  and  3  for  a 
triple  winding,  etc.  When  the  number  of  commutator  bars 
is  exactly  divisible  by  m,  the  windings  will  be  entirely  separate 
from  each  other. 

A  double  lap  winding  will  have  2  X  (2p)  current  paths  be- 
tween brushes,  where  2p  is  the  number  of  poles.  That  is,  each 
winding  for  a  4-pole  machine  will  have  four  current  paths  so 
that  a  double  lap  winding  on  a  4-pole  machine  will  have  eight 
current  paths  between  brushes. 

In  order  that  the  two  sides  of  a  winding  element  or  a  coil 
may  move  simultaneously  under  field  poles  of  opposite 
polarity,  the  total  number  of  coil  sides  that  make  up  the  com- 
plete winding  divided  by  the  number  of  the  field  poles,  that  is 
N  -T-  2p,  is  the  approximate  value  for  both  the  front  and  back 
pitches  as  in  the  case  of  the  single  lap  winding.  Under  such 
conditions  the  electromotive  force  induced  in  the  two  sides  of 
a  winding  element  add  up.  The  smallest  front  or  back  pitch 
to  satisfy  this  condition,  is  the  distance  across  a  single  pole 
face  and  the  largest  front  or  back  pitch  is  the  distance  from 
one  pole  tip  to  the  nearest  pole  tip  of  the  same  polarity. 
If  the  front  and  back  pitches  are  much  less  than  N  -5-  2p, 
then  a  chorded  winding  results. 

Since  y\  and  ?/2  must  be  odd  numbers  and  approximately 
equal  to  N  -f-  2p,  where  2p  is  the  number  of  field  poles,  this 
value  will  help  in  determining  the  value  of  b  to  be  used  in  the 
formula  for  front  pitch  (7/1)  and  back  pitch  (t/2). 

The  multiplex  lap  winding  is  largely  confined  to  windings 
for  small  and  medium  sized  machines  carrying  large  currents 
and  using  coils  made  up  of  wire  rather  than  copper  strips  or 


16  '         ARMATURE  WINDING  AND  MOTOR  REPAIR 

bars.  This  is  for  the  reason  that  in  such  a  winding  for  a 
large  multipolar  machine  the  bars  would  be  too  thin  and  a 
mechanical  construction  result  which  would  make  another 
type  of  winding  advisable,  probably  a  series-parallel  design. 

WAVE— SERIES  OR  TWO-CIRCUIT    WINDING 

The  wave  winding  is  so  called  from  the  zig  zag  or  wave  path 
that  the  winding  takes  through  the  slots  of  the  armature,  as 
shown  in  Figs.  11  and  12.  This  type  of  winding  is  more 


FIG.  11. — Wave  winding  showing  relative  position  of  one  winding  element  or 
coil  of  two  turns  on  the  armature  of  a  4-pole  machine. 

definitely  described  as  a  series  or  two-circuit  winding  because 
of  the  fact  that  half  of  the  armature  coils  or  sections  are  con- 
nected in  series  and  the  two  halves  are  connected  in  parallel. 
This  winding  therefore  has  only  two  current  paths  in  parallel 


FIG.  12. — Winding  diagram,  for  wave  winding  of  Fig.  11  showing  how  the 
finish  end  of  one  coil  is  joined  to  the  commutator  and  connected  to  the  start 
of  the  next  coil  under  the  next  pair  of  poles.  &'  is  a  continuation  of  6. 

between  brushes  regardless  of  the  number  of  poles.  Only  two 
sets  of  brushes  are  required  for  a  machine  of  any  number  of 
poles  but  improved  commutation  is  brought  about  when  the 
number  of  brushes  equals  the  number  of  field  poles.  The  wave 
winding  is  used  in  small  and  medium  sized  machines  where  it 
is  desired  to  keep  the  number  of  coils  as  small  as  possible. 


DIRECT-CURRENT  WINDINGS  17 

Formulas  for  Wave   Winding. — The   following   rules   and 
restrictions  govern  the  assembling  and  use  of  a  wave  winding: 

1 .  The  front  and  back  pitches  (in  winding  spaces  or  coil  sides)  must 
both  be  odd. 

2.  The  front  and  back  pitches  may  be  equal  or  may  differ  by  two 
or  some  multiple  thereof.     The  former  condition  is  usually  the 
case. 

3.  The  front  and  back  pitches  are  of  the  same  sign  since  they  are 
laid  off  in  the  same  direction. 

4.  The  winding  pitch  is  equal  to  the  sum  of  the  front  and  back 
pitches. 

5.  The  commutator  pitch  and  number  of  commutator  bars  must 
not  have  a  common  factor. 

6.  The  number  of  current  collecting  points  or  brushes  on  the 
commutator  is  always  two  for  any  number  of  poles  but  the 
number  of  sets  of  brushes  may  equal  the  number  of  poles. 

7.  The  maximum  emf  between  two  consecutive  coil  sides   (top 
and   bottom)  in  the  same  slot,  is  equal  or  nearly  equal  to  the 
terminal  voltage  of  the  machine. 

8.  The  finish  of  one  coil  is  joined  to  a  commutator  bar  and  to  the 
start  of  another  coil  which  lies  under  the  next  pair  of  poles. 

The  following  formulas  apply  in  wave  winding: 

N  ±  2 
Winding  pitch  =  y  =  yi  +  y2  = 

Front  pitch  =  y\ 
Back  pitch  =  yz 
Number  pairs  of  poles  =  p 

^»u  2/i  +  2/2       K±l 

Commutator  pitch  =  yk  =  — ~ — 

4  P 

N 
Number    commutator    bars  =  K  =  -^=pXyk±^-' 

N  ±  2 
The  number  of  coil  sides  (N)  must  be  such  that  

will  be  an  even  number  with  y\  and  y%  both  odd  -numbers. 
yi  and  y2  are  usually  taken  equal.  Under  these  conditions 
the  back  pitch  (y2)  is  nearly  equal  to  the  pole  pitch  and  the 
sum  of  the  front  and  back  pitches  is  nearly  equal  to  double 
the  pole  pitch.  If  y2  is  reduced  then  yi  must  be  increased  so 
that  7/1  +  2/2  will  be  constant.  This  is  what  happens  in  a 
chorded  or  fractional  pitch  winding. 


18 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


The  current  (i)  which  flows  in  each  conductor  of  a  wave 
winding  is  always  one-half  the  total  armature  current,  i  = 
I  -f-  2.  This  value  of  (i)  should  be  considered  when  select- 
ing the  proper  size  of  conductors  for  coils  of  wave  windings. 
The  closed  winding  formula  for  a  wave  winding  is  yk  = 
(K  ±  a)  -f-  p.  Where  yk  is  the  commutator  pitch,  K  the  num- 


FIG.  13. — Wave  winding  for  a  4-pole  machine  showing  the  direction  of 
current  flow  in  one  of  the  two  current  paths  for  the  brush  position  as  illus- 
trated. 

ber  of  commutator  bars,  a  the  pairs  of  parallel  circuits  in  the 
winding  and  p  the  number  of  pairs  of  poles.  When  a  =  1,  it 
is  only  possible  to  make  a  single  winding.  The  highest  com- 
mon factor  of  yk  and  K  gives  the  type  of  winding,  whether 
single,  double,  etc.  The  number  of  coil  sides  in  a  wave  wind- 
ing equals  twice  the  number  of  commutator  bars. 

The  number  of  slots  without  idle  coils  must  satisfy  the 
formula,  s  =  (2K)  -i-  N8.  Where  K  is  the  number  commu- 
tator bars  and  Na  the  number  of  coil  sides  per  slot. 

A  wave  winding  is  symmetrical  when  K  -f-  a,  s  -f-  a  and 
p  -5-  a,  are  whole  numbers.  Where  K  is  the  number  of  com- 


DIRECT-CURRENT  WINDINGS  19 

mutator  bars,  a,  the  number  of  pairs  of  parallel  circuits  in 
the  winding,  s  the  number  of  slots,  and  p  the  number  of  pairs 
of  poles. 

When  equalizer  rings  arc  needed  with  a  wave  winding,  the 
use  of  one  ring  to  every  15  to  20  commutator  bars  is  good 
practice. 

Multiplex  Wave  or  Series-Parallel  Winding. — In  a  single 
wave  or  series  winding  the  number  of  armature  circuits  in 
parallel  is  always  equal  to  two,  while  in  a  lap  or  multiple  wind- 
ing the  number  of  circuits  in  parallel  is  equal  to  the  number  of 
field  poles.  The  series-parallel  winding  is  a  design  of  wave 
winding  in  which  the  number  of  armature  circuits  in  parallel 
may  be  larger  than  two  and  yet  smaller  than  the  number  of 
field  poles.  It  is  especially  suitable  for  large  multipolar  arma- 
tures which  use  winding  elements  made  up  of  copper  bars. 

Formulas  for  Series-Parallel  Winding. — The  following 
rules  govern  the  assembling  and  use  of  this  winding: 

1.  The  front  and  back  pitches  (in  winding  spaces  or  coil  sides) 
must  be  odd  numbers.     They  may  be  equal  to  or  different  from 
each  other  although  they  are  usually  equal. 

2.  The  winding  should  be  symmetrical  as  shown  by  the  number 
of  field  poles  divided  by  the  number  of  armature  circuits  in 
parallel  being  a  whole  number.     If  this  is  not  the  case  the  final 
winding  should  be  made  up  of  a  sufficient  number  of  independent 
windings  to  make  the  number  of  field  poles  divided  by  the  number 
of  armature  circuits  in  parallel  a  whole  number  for  each  of  the 
independent  windings. 

3.  It  is  necessary  that  the  number  of  commutator  bars  divided  by 
one-half  the  number  of  armature  circuits  in  parallel  shall  be 
a  whole  number.     If  the  conditions  of  (2)   are  fulfilled,  the 
condition  named  here  is  likewise  satisfied  at  the  same  time. 

4.  If  the  number  of  commutator  bars  and  the  commutator  pitch 
have  no  common  factor,  the  winding  is  a  single  one;  if  the  com- 
mon factor  is  2,  the  winding  is  double  or  if  it  is  equal  to  m,  the 
winding  is  multiplex  of  m  separate  windings.     The  common 
factor  must,  however,  be  smaller  than  half  the  number  of  circuits. 
An  illustration  will  bring  out  these  conditions.     In  the  case  of 
an  8-pole  machine  with  27  slots,  54  commutator  bars,  4  Con- 
ductors per  slot,  and  a  commutator  pitch  of  13,  the  two  series 
windings  are  singly  closed  forming  one  winding,  since  54  and  1 3 


20  ARMATURE  WINDING  AND  MOTOR  REPAIR 

have  no  common  factor.  For  a  machine  of  4-poles,  with  20  slots, 
40  commutator  bars,  4  conductors  per  slot,  commutator  pitch  of 
18,  the  winding  may  be  made  up  of  two  series  windings,  the 
•  values  40,  18  and  4  have  a  common  factor  of  2.  This  makes  the 
winding  doubly  closed  and  consists  of  two  series  windings  with 
each  in  turn  consisting  of  two  other  series  windings,  each  of  the 
latter  forming  one  continuous  winding. 

5.  For  large  machines  using  series-parallel  windings,  it  has  been 
found  advisable  in  most  cases  to  use  equipotential  connect- 
ions and    connect  about  every  fourth  or  eighth  commutator 
section.     (See   heading  " Equipotential  Connectors"  on  page 
21.) 

6.  The  number  of  current  collecting  points  or  brushes  on  the  com- 
mutator is  equal  to  the  number  of  armature  sections  in  parallel. 

The  following  formulas  apply  in  the  case  of  a  series-parallel 
winding : 

Front  pitch  =  yi 
Back  pitch  =  y2 

Winding  pitch  =  y  =  yl  +  y2  = 

Commutator  pitch  =  yk  =  - 

N 
Number  commutator  bars  =  K  =  -^ 

z 

Where  p  is  number  of  pairs  of  poles  and  N  is  the  number 
of  coil  sides  in  the  winding.  N  must  be  such  a  value  that 

N  ±  2a. 

-  is  a  whole  number. 
P 

Symmetrical  Windings.— All  armature  windings  should 
be  made  symmetrical  if  possible.  A  winding  is  symmetrical 
when  tLe  total  number  of  slots  divided  by  one-half  the  number 
of  armature  sections  in  parallel  is  a  whole  number.  If  this 
is  not  the  case  the  emfs  induced  in  different  circuits  will  pro- 
duce circulating  currents.  A  winding  is  also  symmetrical 
if  the  number  of  field  poles  divided  by  the  number  of  armature 
sections  in  parallel  is  a  whole  number  or  the  number  of  com- 
mutator bars  is  divisible  by  one-half  the  number  of  armature 
sections  in  parallel.  To  be  sure  that  any  winding  is  symmetri- 
cal all  three  of  these  conditions  should  be  fulfilled. 


DIRECT-CURRENT  WINDINGS  21 

Possible  Symmetrical  Windings  for  D.-C.  Machines  of 
Different  Number  of  Poles. — In  a  discussion  on  armature 
windings,  Stanley  Parker  Smith  (London  Electrician,  June  16, 
1916)  has  selected  the  following  as  the  most  suitable  windings 
for  machines  of  different  number  of  poles: 

1.  For  a  two-pole  machine  either  a  two-circuit  lap  or  wave  winding 
can  be  used.     The  former  is  usually  preferred. 

2.  For  a  four-pole  machine  either  a  two-circuit  wave  or  a  four- 
circuit  lap  winding  can  be  used. 

3.  For  a  six-pole  machine  we  are  limited  to  the  two-circuit  wave 
and  six-circuit  lap  windings,  since  the  four-circuit  wave  wind- 
ing (a  =  2,  p  =  3)  is  unsymmetrical.     This  is  probably  the 
most  objectionable  restriction  of  all,  because  so  many  cases 
arise  where  four  circuits  are  desirable  in  a  six-pole  machine. 

4.  For  an  eight-pole  machine  wave  windings  with  two  or  four 
circuits  and  an  eight-circuit  lap  winding  can  be  used.     This 
is    the   first   instance    of   a  symmetrical  wave  winding  with 
a  >  1<  p,  and  can  be  used  with  great  advantage  in  many  cases. 

5.  For  a  10-pole  machine  there  are  again  only  two  symmetrical 
windings — the  two-circuit  wave  and  10-circuit  lap  windings. 

6.  For  a  12-pole  machine  the  possibilities  are  much  greater,  for 
wave  windings  with  two,  four  or  six  circuits  and  a  lap  winding 
with  12  circuits  can  be  used. 

7.  The  list  can  be  continued  further,  if  desired,  but  it  is  plainly 
seen  that  certain  very  important  advantages  are  open  to  the 
designer  by  making  use  of  wave  windings  with  more  than  two 
circuits    in    8,    12    and    16-pole    machines.     It   is   necessary, 
however,  to  observe  the  other  conditions  of  symmetry,  namely 
s/a  must  be  a  whole  number  and  idle  coils  must  be  avoided. 

Equipotential  Connectors  (Equalizing  Rings  and  Phase 
Tappings). — In  a  symmetrical  winding,  that  is,  a  winding 
with  identical  K/a  phase  systems,  there  are  always  a  coils 
at  the  same  potential,  and  these  can  be  joined  together  if 
desired.  If  no  dissymmetry  whatever  were  present,  however, 
there  would  be  little  object  in  making  such  connections,  unless 
they  are  needed  as  phase  tappings  to  obtain  an  alternating 
pressure.  Actually  there  are  many  cases  of  dissymmetry  in 
a  machine,  apart  from  those  due  to  the  arrangement  of  the 
winding  already  mentioned.  Thus,  the  magnetic  material 
may  not  be  uniform,  the  pole-shoes  may  not  be  properly 


22  ARMATURE  WINDING  AND  MOTOR  REPAIR 

spaced,  the  gap  may  not  be  uniform,  and  so  on.  In  general, 
perfect  symmetry  must  be  regarded  as  an  unattainable  ideal 
in  practice  according  to  Mr.  Smith,  who  calls  attention  to  the 
following  conditions: 

Owing  to  these  dissymmetries,  the  pressure  induced  in  the 
several  armature  circuits  varies,  and  causes  equalizing  currents 
to  flow  through  the  brushes.  These  equalizing  currents,  if  of 
sufficient  strength,  produce  sparking  and  in  any  case,  load  the 
brushes  in  an  undesirable  manner.  It  is  interesting  to  note 
that,  even  when  no  equalizing  rings  are  present,  the  positive 
and  negative  collecting  rings  act  as  equalizers,  and  the  tend- 
ency of  these  equalizing  currents  is  to  neutralize  the  inequali- 
ties in  the  magnetic  field  to  which  they  are  due.  When 
equalizing  rings  are  used,  however,  large  equalizing  currents 
will  flow  along  these  and  strongly  damp  out  any  inequalities 
in  the  field,  and  so  reduce  the  difference  of  potential  between 
corresponding  points  in  the  winding.  Consequently  the 
brushes  are  relieved,  and  are  so  much  better  able  to  perform 
their  proper  function  of  collecting  the  current. 

Equalizing  rings  must  not  be  regarded  as  in  any  way  es- 
sential, and  many  machines  work  quite  well  without  them. 
Nevertheless,  they  add  a  certain  factor  of  safety  which  the 
manufacturer  is  often  glad  to  purchase  at  so  small  a  cost,  for 
he  is  not  only  surer  of  his  machine  passing  the  test  satis- 
factorily but  also  knows  that  after-effects,  like  wear  of  the 
bearings,  cannot  give  rise  to  such  serious  trouble  as  when  equal- 
izers are  absent.  Consequently,  equalizers  are  seen  on  many 
large  machines  with  lap-wound  armatures,  or  with  wave 
windings  with  more  than  two  circuits.  Equalizing  rings 
should  not  have  an  extremely  low  resistance.  Such  practice 
requires  not  only  an  excessive  amount  of  copper,  but  leads  to 
considerable  loss  and  heating  in  the  winding.  All  that  is 
really  necessary  is  to  provide  an  alternative  path  of  negligible 
resistance  compared  with  that  of  the  brushes,  and  for  this 
purpose  it  is  usually  sufficient  to  make  the  section  of  the  rings 
about  half  that  of  the  conductors. 

Regarding  the  number  of  equalizing  rings,  much  depends 
on  the  opinion  of  the  designer.  With  lap  windings,  one  ring 
for  every  6  to  12  segments  is  common  practice,  but  this  is 


DIRECT-CURRENT  WINDINGS  23 

scarcely  feasible  when  p  >  a,  for  here  the  potential  pitch, 1 
yp  =  K/a,  may  be  fairly  large  and  the  number  of  rings  be- 
comes prohibitive.  In  such  cases  (wave  windings  with  more 
than  two  circuits),  one  ring  for  every  15  to  20  segments  may 
suffice.  There  is  no  need  to  make  the  pitch  between  all  the 
rings  the  same,  but  designers  generally  prefer  to  split  up  yp 
into  a  whole  number  of  parts.  In  this  case  the  tappings  form 
a  symmetrical  polyphase  system  of  pressures. 

Best  D.-C.  Windings  for  a  Repair  Shop  to  Use. — To  secure 
good  commutation  and  eliminate  heating  troubles,  all  windings 
should  be  made  symmetrical.  This  among  other  things 
means  that  there  must  not  be  more  armature  circuits  in  paral- 
lel than  there  are  field  poles.  Where  the  number  of  armature 
circuits  equals  the  number  of  poles  either  the  lap  or  the  wave 
winding  may  be  used.  The  lap  winding  is  however  mostly 
used  in  such  a  case.  The  wave  winding  then  need  only  be 
considered  in  those  cases  where  the  number  of  armature  circuits 
is  to  be  less  than  the  number  of  field  poles.  A  single  winding 
is  more  simple  and  should  be  used  in  preference  to  a  compli- 
cated one  where  conditions  permit  either  a  single  winding 
or  two  or  more  separate  windings  except  in  those  cases  where 
a  very  heavy  current  is  to  be  carried  in  the  armature.  In 
general  then,  where  changes  in  armature  windings  are  neces- 
sary that  call  for  a  choice  of  types  of  windings,  it  is  advisable 
to  employ  either  a  lap  winding  with  as  many  circuits  as  there 
are  poles  or  a  wave  winding  with  two  circuits.  Usually  a  lap 
winding  is  possible  if  the  number  of  coils  is  a  common  multiple 
of  the  number  of  poles  and  the  number  of  slots.  The  number 
of  coils  is  equal  to  the  number  of  commutator  bars. 

Number  of  Armature  Slots. — The  open  slot  with  parallel 
sides  is  most  used  for  large  armatures  so  that  form  wound 
coils  can  be  easily  assembled.  These  coils  are  held  in  place 
by  wire  bands  or  by  wood  or  fiber  wedges  driven  into  grooves 
at  the  tips  of  the  teeth  as  shown  in  Chapter  III.  The  number 
of  slots  per  pole  is  usually  not  less  than  10.  For  multipolar 
armatures  there  are  at  least  from  three  to  four  slots  in  the  space 

1  The  potential  pitch  is  the  number  of  commutator  bars  between  suc- 
cessive equipotential  points  in  the  winding  which  can  be  joined  together 
when  equalizing  rings  are  needed. 


24  ARMATURE  WINDING  AND  MOTOR  REPAIR 

between  pole  tips.  In  the  case  of  high  speed  machines  having 
large  pole  pitch  there  may  be  from  14  to  18  slots  per  pole. 
For  machines  above  say  5  hp.  the  area  of  the  slot  will  approx- 
imate one  square  inch.  A  rough  rule  for  the  capacity  of 
a  slot  of  this  area  is  about  1,000  amp.  turns  for  machines 
under  500  volts. 

Voltage  between  Commutator  Segments. — For  direct-cur- 
rent machines  without  interpoles,  about  15  volts  between 
segments  of  the  commutator  is  the  maximum  allowed  while 
double  this  value  can  be  considered  as  the  maximum  for  ma- 
chines employing  commutating  interpoles.  This  value  of 
voltage  between  segments  can  be  obtained  for  an  existing 
machine  by  measuring  this  voltage  between  plus  and  minus 
brushes  and  dividing  by  the  number  of  commutator  segments 
between  these  brushes.  The  voltage  between  a  positive  and 
nagative  brush  will  depend  upon  the  number  of  inductors  (wires 
making  up  one  side  of  a  coil)  connected  in  series  between  the 
brushes.  For  calculating  windings  for  machines,  Prof.  Still 
gives  the  following  relationships  between  machine  voltage 
and  volts  between  commutator  segments  for  good  operation: 

VOLTAGE  BETWEEN  SEGMENTS  OF  COMMUTATORS 


Machine  voltage 

Volts  between  commutator  segments 

110 

Ito6 

220 

2.5  to  10 

600 

5  to  18 

1200 

9  to  25 

Number  of  Commutator  Bars.^-The  proper  number  of 
commutator  bars  for  a  particular  winding  depends  on  the  volt- 
age between  commutator  bars.  The  number  of  bars  may  bev 
a  multiple  of  the  number  of  slots.  For  low  voltage  machine 
there  may  be  one,  two  or  three  bars  per  slot  while  in  higher 
voltage  slow  speed  machines,  there  may  be  as  many  as  four 
or  five  bars  per  slot.  While  improved  commutation  results 
from  a  large  number  of  commutator  bars  the  difficulty  in 
repair  and  assembly  from  a  mechanical  standpoint  offset 
their  advantage  over  fewer  bars  for  the  same  conditions  of 
winding  and  machine  operation. 


DIRECT-CURRENT  WINDINGS 


25 


Usual  Speeds  and  Poles  for  Different  Sizes  of  Generators. — 

For  direct  current  generators  Prof.  Still1  gives  the  following 
relationship  between  number  of  poles  and  speed  for  different 
ratings  of  machines.  This  table  represents  usual  practice 
by  designers. 

NUMBER  OF  POLES  AND  USUAL  SPEED  LIMITS  OF  D.-C.  GENERATORS 


Output  in  kw. 

Number  of  poles 

Speed  in  rpm, 
maximum  and  minimum 

Oto  10 

2 

2400  to  600 

10  to  50 

4 

1300  to  350 

50  to  100 

4  or  6 

1100  to  230 

100  to  300 

6  or  8 

700  to  160 

300  to  600 

6  or  10 

500  to  120 

600  to  1000 

8  or  12 

400  to  100 

1000  to  3000 

10  or  20 

200  to  70 

Safe  Armature  Speeds. — The  safe  speed  of  an  armature 
varies  with  the  armature  construction.  Direct-current  ma- 
chines are  built  with  a  peripheral  speed  of  2,500  to  3,500  feet 
per  minute.  It  is  not  advisable  to  exceed  an  armature  periph- 
eral speed  of  6,000  feet  per  minute  in  machines  which  are 
not  designed  to  take  care  of  the  mechanical  stresses  incident 
to  the  higher  speeds. 

1  Principles  of  Electrical  Design,  page  81. 


CHAPTER  II 
ALTERNATING -CURRENT  WINDINGS 

In  the  main,  the  windings  for  alternating-current  motors 
and  generators  are  alike.  For  this  reason  details  of  the  wind- 
ings that  can  be  used  for  either  machines  will  be  given.  It 
should  be  noted  at  this  point  that  direct-current  and  alter- 
nating-current windings  differ  essentially  by  the  former  being 
of  the  closed-circuit  type  while  most  alternating-current  wind- 
ings are  of  the  open-circuit  type.  Either  open-  or  closed- 
circuit  types  of  windings  may  however  be  employed  in  alter- 
nating-current machines,  but  the  open  type  is  in  most  common 
use.  By  a  closed  winding  is  meant  one  which  has  a  continuous 
path  through  the  armature  and  re-enters  itself  to  form  a 
closed  circuit.  Such  a  direct-current  winding  always  has  at 
least  two  current  paths  between  brushes.  In  the  case  of  an 
open-circuit  alternating -circuit  winding,  there  is  a  continuous 
path  through  the  conductors  of  the  coils  of  each  phase  of  the 
winding,  with  the  ends  of  this  path  forming  two  free  ends. 
Such  a  winding  does  not  close  on  itself. 

In  the  closed-circuit  windings  of  direct-current  machines, 
the  bars  of  the  commutator  are  simply  connected  at  equally 
distant  points  around  the  winding.  In  the  open-circuit  wind- 
ing of  an  alternating-current  generator  with  revolving  arma- 
ture, the  terminals  of  the  completed  winding  are  connected  to 
collector  rings  and  the  winding  is  open-circuited  until  closed 
by  the  connections  between  the  brushes.  Closed-circuit  wind- 
ings are  only  used  for  special  alternating-current  machines. 
The  conditions  which  usually  call  for  a  closed-circuit  wind- 
ing are  the  following: 

1.  An  alternating-current  machine  which  must  deliver  a  large 
current  at  low  voltage.  In  such  a  case  the  winding  usually 
consists  of  several  similar  paths  connected  in  parallel  to  the 

26 


ALTERNATING-CURRENT  WINDINGS  27 

terminals  of  the  armature,  thus  forming  one  or  more  closed  cir- 
cuits within  the  armature  winding. 

2.  In  designs  of  machines  that  must  handle  direct  and  alternating 
current,  as  in  double  current  generators  and  in  rotary  converters. 

In  general  it  may  be  said  that  a  direct -current  winding  may 
be  changed  so  that  the  machine  can  be  used  as  an  alternator, 
but  an  alternating-current  winding  cannot  be  used  for  a  direct 
current  generator  since  it  is  not  reentrant. 

Types  of  A.-C.  Windings. — With  reference  to  the  arrange- 
ments of  coils  used  in  an  alternating-current  armature,  wind- 
ings may  be  divided  into  two  general  classes  as  follows: 

I.  Distributed  Windings. 

1.  Spiral  or  chain. 

2.  Lap. 

3.  Wave. 

II.  Concentrated  Windings. 

1.  Lap. 

2.  Wave. 

Distributed  Windings. — An  armature  winding  which  has 
its  inductors  of  any  one  phase  undigr  a  single  pole  placed  in 
several  slots,  is  said  to  be  distributed.  When  these  inductors 
are  Dunched  together  in  one  slot  per  pole,  per  phase,  the 
winding  is  called  concentrated.  It  is  usual  in  a  distributed 
winding  to  distribute  the  series  inductors  in  any  phase  of  the 
winding  among  two  or  more  slots  under  each  pole.  This 
tends  to  diminish  armature  reactance  and  gives  a  better  emf . 
wave,  besides  offering  a  better  distribution  of  the  heating  due 
to  armature  copper  loss  than  in  concentrated  windings. 

Concentrated  Windings. — The  uni-slot  or  concentrated 
winding  gives  the  largest  possible  emf  from  a  given  number  of 
inductors  in  the  winding.  That  is  for  a  definite  fixed  speed 
and  field  strength  in  an  alternator,  the  concentrated  winding 
requires  a  less  number  of  inductors  than  a  distributed  winding, 
but  increases  the  number  of  turns  per  coil. 

Spiral  or  Chain  Winding. — In  this  winding  as  shown  in 
Fig.  14  there  is  only  one  coil  side  in  a  slot.  An  odd  or  even 
number  of  inductors  per  slot  may  be  used  but  several  shapes 
of  coils  are  required  since  the  coils  enclose  each  other  and  must 


28 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


have  special  end  shapes  to  clear  each  other.  This  arrange- 
ment, however,  makes  possible  good  insulation  of  end  con- 
nections through  adequate  separating  air  spaces  in  high  vol- 
tage machines.  The  number  of  coils  required  in  this  winding 


D  - 

| 

1 

f  j 

> 

H 

y,, 

x 

/ 

| 

> 

/ 

\ 

/ 

y 

( 

f 

7 

* 

f 

1  - 

' 

FIG.  14. — A  spiral  or  chain  winding  for  a  2-phase,  4-pole  machine  with 
the  coils  of  one  phase  in  place  and  connected.  The  coils  of  the  other  phase 
go  in  the  slots  shown  by  the  full  lines  inside  the  coils. 

is  also  small  compared  with  other  windings.     This  type  of 
winding  is  mainly  used  in  alternating-current  generators. 

Lap  and  Wave  Windings.- — Both  distributed  and  concen- 
trated windings  make  use  of  lap  and  wave  connections.  These 


FIG.  15. — Single-phase  lap  winding  for  the  same  conditions  as   the  wave 
winding  in  Fig.  16. 


FTG.  16. — Single-phase  distributed  wave  winding  with  two  slots  per  pole  per 
phase  and  one  coil  side  per  slot. 

arrangements  are  in  principle  the  same  as  used  in  direct-current 
windings.  (See  Chapter  I,  pages  9  and  16.)  The  diagrams  of 
Figs.  15  and  16  show  single-phase  distributed  lap  and  wave 


ALTERNATING-CURRENT  WINDINGS 


29 


windings  for  a  four-pole  armature,  having  two  slots  per  pole 
with  one  inductor  per  slot. 

In  a  double-layer  lap  or  wave  winding  for  an  alternating- 
current  armature,  the  coils  are  usually  of  the  same  general 
shape  as  used  in  direct-current  windings.  Instead  of  con- 
necting the  terminals  of  the  coils  to  a  commutator,  they  are 
connected  together  in  a  definite  order  (see  Chapter  XI,  page 
288)  for  each  phase.  The  phase  windings  are  then  connected 
together  in  star  or  delta  as  shown  in  Fig.  22.  In  that  diagram 
a  three-phase  wave  winding  is  shown  for  a  four-pole  machine. 
In  the  double-layer  winding,  the  number  of  conductors  per 


FIG.  17. — At  the  left  is  shown  the  appearance  of  one  formed  coil  in  a  2- 
layer  winding.  One  side  of  the  coil  lies  in  the  top  of  the  slot  and  the  other 
side  in  the  bottom  of  the  slot.  The  top  side  bears  an  odd  number  and  the 
bottom  side  an  even  number.  The  coil  pitch  in  this  case  is  6  slots  or  13 
winding  spaces.  At  the  right  the  method  of  connecting  coils  for  a  2-layer 
winding  using  form  coils  is  shown.  The  finish  (F)  of  one  coil  is  joined 
to  the  start  (S)  of  the  next. 

slot  must  be  a  multiple  of  two.  It  lends  itself  to  a  variety 
of  connections,  particularly  to  a  fractional  pitch  lap  winding 
where  the  two  sides  of  a  coil  are  not  similarly  placed  in  respect 
to  the  center  lines  of  the  poles.  The  double-layer  winding 
is  not,  however,  as  well  suited  for  high  voltages  as  the  single- 
layer  winding.  For  this  reason  many  water-wheel  types  of 
generators  have  been  built  with  a  single-layer  winding,  which 
in  its  most  common  form,  is  known  as  the  spiral  or  chain 
winding. 

When  laying  out  or  changing  a  double-layer  winding,  it 
is  usual  to  assign  odd  numbers  to  the  sides  of  coils  in  the  top 
of  the  slots  and  even  numbers  to  the  sides  in  the  bottom  of 
the  slots.  This  is  important  when  the  pitch  of  the  armature. 


30  ARMATURE  WINDING  AND  MOTOR  REPAIR 

coils  is  expressed  in  terms  of  coil  sides  (winding  spaces)  in- 
stead of  slots. 

Whole-coiled  and  Half-coiled  Windings. — When  the  coils 
of  an  alternating  current  winding  are  connected  so  that  there 
are  as  many  coils  per  phase  as  there  are  poles,  the  winding  is 
called  "whole-coiled."  When  the  coils  are  connected  so  that 
there  is  only  one  coil  per  phase  per  pair  of  poles,  the  winding  is 
called  "  half-coiled."  The  main  difference  between  these  two 
connections  is  in  the  method  of  making  the  end  connections 


A — Whole-coiled  winding  B — Half-coiled  winding 

FIG.  18. — A  6-pole  stator  with  whole-coiled  and  half-coiled  windings. 
The  whole-coiled  winding,  A,  has  as  many  coils  per  phase  as  there  are  poles. 
The  half-coiled  winding  B  has  only  one  coil  per  phase  per  pair  of  poles. 

for  the  coils.  In  the  "  whole-coiled "  winding  each  slot  con- 
tains two  coil  sides.  It  is  not,  however,  strictly  a  double- 
layer  winding,  as  the  coil  sides  are  placed  side  by  side  and 
not  one  above  the  other.  In  the  "  half  -coiled"  winding,  how- 
ever, each  coil  may  have  twice  the  number  of  turns  of  a  " whole- 
coiled"  winding  or  the  two  coils  under  a  north  or  south  pole 
of  the  latter  type  may  be  connected  in  series  and  taped  to- 
gether to  form  one  coil  in  case  of  a  change  in  connections. 

The  "  half  -coiled "  winding  has  the  advantage  that,  when 
used  with  large  generators  the  armature  frame  may  be  split 
into  two  sections  for  shipment  or  repair,  without  disturbing 
many  of  the  end  connections. 

Single-phase  and  Polyphase  Windings. — The  winding  of 
a  single-phase  motor  or  generator  has  only  one  group  of  induc- 
tors per  pole,  placed  in  one  slot  or  several  slots  depending 
upon  whether  or  not  the  winding  is  concentrated  or  distrib- 


ALTERNATING-CURRENT  WINDINGS 


31 


FIG.  19. — A  simple  single-phase  winding. 


FIG.  20. — Simple  2-phase  winding. 


FIG.  21. — Simple  3-phase  winding. 


FIG.  22. — A  3-phase  winding  showing  how  it  may  be  connected  in  delta 

or  star. 


32  ARMATURE  WINDING  AND  MOTOR  REPAIR 

uted.  Such  a  single-phase  concentrated  wave  winding  for 
a  four-pole  armature  is  shown  in  Fig.  19. 

Two-phase  and  three-phase  windings  may  be  considered 
as  made  up  of  single-phase  windings  properly  placed  on  the 
same  armature.  For  the  two-phase  windings  two  separate 
single-phase  windings  are  used  spaced  90  electrical  degrees 
apart.  This  is  shown  in  Fig.  20.  For  the  three-phase  wind- 
ing, three  single-phase  windings  are  used,  spaced  120  degrees 
apart,  as  illustrated  in  Fig.  21.  Although  the  single-phase 
windings  are  independent  of  each  other,  their  terminals  are 
connected  in  star  or  delta  as  shown  in  Fig.  22. 

Coil  Pitch. — In  the  case  of  a  two-phase  winding,  the  total 
number  of  slots  should  be  just  divisible  by  two  so  that  each 
phase  will  have  the  same  number  of  winding  elements  or 
coils  per  pole.  In  the  same  way,  for  a  three-phase  winding 
the  total  number  of  coil  sides  or  the  total  number  of  slots 
should  be  just  divisible  by  three  (the  number  of  phases) 
and  sometimes  by  the  number  of  poles.  This  will  result  in 
a  full  pitch  winding,  that  is,  a  winding  in  which  a  coil  spans 
exactly  the  distance  between  the  centers  of  adjacent  poles. 
If  the  coil  spans  less  than  this  distance,  so  that  its  two  sides 
are  not  exactly  under  the  centers  of  adjacent  poles  at  the  same 
time,  it  is  said  to  have  a  fractional-pitch.  When  a  fractional- 
pitch  is  used  in  alternators  on  account  of  the  electrical  factors 
of  the  design,  such  as  to  secure  as  nearly  as  possible  a  sine 
wave  shape  of  emf,  the  total  number  of  slots  per  phase  must 
be  a  whole  number.  A  fractional-pitch  is  also  widely  used 
in  induction  motors. 

Coil  pitch  is  expressed  as  a  fraction  of  the  pole  pitch,  in 
slots,  in  electrical  degrees  or  in  winding  spaces  (coil  sides). 
In  the  case  of  a  six-pole  machine  having  72  stator  slots,  and 
a  double-layer  winding,  the  pole  pitch  would  be  12  slots.  If 
the  coil  pitch  were  given  as  %,  this  would  be  120  degrees 
or  eight  slots  or  13  winding  spaces  (coil  sides).  A  full  coil 
pitch  for  this  winding  would  be  180  degrees,  12  slots  or  21 
winding  spaces. 

Phase  Spread  of  Windings. — The  spread  or  space  occupied 
by  each  single-phase  winding  is  known  as  the  phase  spread 
of  the  winding.  For  a  two-phase  winding  the  phase  spread 


ALTERNATING-CURRENT  WINDINGS  33 

is  (180  -r-  2)  or  90  degrees.  For  a  three-phase  winding,  it  is 
(180  -T-  3)  or  60  degrees.  In  a  single-phase  winding,  the  phase 
spread  is  theoretically  180  degrees.  Prof.  Alfred  Still  points 
out,  however,  in  his  book  on  "  Principles  of  Electrical  Design, " 
that  nothing  is  gained  by  winding  all  the  slots  on  the  armature 
surface  of  a  single-phase  machine.  After  a  certain  width  of 
winding  has  been  reached  the  filling  of  additional  slots  merely 
increases  the  resistance  and  inductance  of  the  winding  with- 
out any  appreciable  gain  in  the  developed  voltage.  In  prac- 
tice only  about  75  per  cent,  of  the  available  slot  space  is 
utilized  making  the  phase  spread  for  a  single-phase  winding 
about  135  electrical  degrees. 

The  fact  that,  in  polyphase  machines,  the  whole  of  the  arma- 
ture surface  is  available  for  the  winding,  while  only  a  portion 
is  utilized  in  a  single-phase  alternator,  accounts  for  the  fact 
that  the  output  of  the  latter  is  less  than  that  of  the  polyphase 
machine  using  the  same  size  of  frame.  In  a  three-phase 
machine  it  is  only  necessary  to  omit  one  of  the  phase  windings 
entirely  and  connect  the  two  remaining  phases  in  series  to 
obtain  a  single-phase  generator.  Such  a  modified  generator 
will  give  about  two-thirds  of  the  output  of  the  polyphase 
connection.  A  three-phase  star  connected  induction  motor 
can  also  be  used  as  a  single-phase  motor  by  properly  con- 
necting two  phases  of  it  (see  Chapter  XI). 

Two-phase  from  Four -phase  Windings. — In  many  cases  the 
two-phase  induction  motor  is  designed  as  a  four-phase  machine 
with  the  connections  between  conductors  of  the  winding 
arranged  so  as  to  permit  operation  on  a  two-phase  supply 
circuit.  As  shown  in  Fig.  23,  a  two-phase  winding  maybe 
secured  from  a  four-phase  grouping  of  coils  by  connecting  the 
first  and  third  groups  in  series  and  the  second  and  fourth 
groups  in  series. 

Three-phase  from  Six-phase  Windings. — Few  strictly 
three-phase  induction  motors  are  built.  The  design  may  be 
more  properly  called  a  six-phase  winding  with  the  three  phases 
'  spaced  120  electrical  degrees  and  the  connections  of  coils 
such  as  to  permit  the  motor  to  be  operated  on  a  three-phase 
circuit.  As  shown  in  Fig.  24  in  a  six-phase  winding  the  coils 
of  the  six-phases  are  spaced  60  electrical  degrees.  For  three- 


34 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


phase  operation  the  coils  of  phases  one  and  four,  three  and 
six,  and  five  and  two  are  connected  in  series.  The  terminals 
of  phases  two,  four  and  six  are  connected  to  a  common  point. 


FIG.  23. — Connections  for  four  windings  spaced  90  electrical  degrees  apart 
to  secure  a  2-phase  motor.  There  are  really  four  phases  shown,  the  first  and 
third  and  the  second  and  fourth  are  connected  in  series.  The  full  lines  are 
front  connections  of  coils  and  dotted  lines  the  back  connections.  $  and  F 
indicate  the  start  and  finish  of  the  different  groups  of  coils. 


FIG.  24. — Connections  for  a  6-phase  winding  design  so  that  a  3-phase 
winding  is  secured.  The  six  windings  are  spaced  60  electrical  degrees 
apart.  In  this  diagram  phases  one  and  four,  three  and  six,  and  five  and  two 
are  connected  in  series  and  the  terminals  F2,  F4  and  Fe  jointed  to  a  common 
point.  The  3-phase  leads  are  Si,  S3,  and  SB.  The  full  lines  indicate 
front  connections  and  dotted  lines  back  connections.  S  and  F  indicate  the 
start  and  finish  of  the  different  groups  of  coils. 

Wire,  Strap  and  Bar  Wound  Coils. — For  the  coils  used  in 
small  motors  round  insulated  wire  is  most  employed.  These 


ALTERNATING-CURRENT  WINDINGS  35 

coils  are  either  wound  in  the  slots  by  hand  or  assembled  by 
use  of  specially  formed  coils  wound  in  forms  and  insulated 
before  being  placed  in  the  slots.  Such  formed  coils  are  usu- 
ally used  except  in  cases  where  the  slots  are  closed  or  nearly 
closed.  For  further  information  on  formed  coils  see  page  4, 
Chapter  I,  and  page  141,  Chapter  VI. 

For  large  motors  and  generators  where  the  amperes  to  be 
carried  in  each  armature  circuit  is  a  large  value,  copper  straps 
are  frequently  employed  for  making  up  the  armature  coils. 
In  very  large  machines  a  copper  bar  is  used  instead  of  the 
copper  straps.  In  such  a  case  one  bar  serves  as  the  inductor 
of  a  coil  having  one  turn  per  slot.  A  two-layer  bar  winding 
made  up  of  two  bars  per  coil  and  four  bars  per  slot  is  also 
used.  The  copper  bars  are  connected  to  the  end  connections 
of  the  coils  by  brazing,  welding  or  bolting.  In  all  cases, 
whatever  the  construction  of  the  coil  used,  the  slots  must  be 
properly  insulated  with  fullerboard,  mica,  fish  paper  or  other 
suitable  insulating  material.  For  data  on  slot  insulation  see 
Chapter  VII. 

The  current  density  in  alternating-current  windings  is 
about  2500  amp.  per  square  inch  of  armature  conductor  in 
small  machines,  2000  amp.  in  medium  sizes  and  1500  amp.  in 
high-voltage  designs.  Except  in  high-speed  machines  it  is 
not  safe  to  use  the  maximum  limit  owing  to  damage  to  wind- 
ings from  over  heating. 

METHODS  FOR  LAYING  OUT  AND  CONNECTING  ALTERNATING- 
CURRENT  WINDINGS 

In  rewinding  an  alternating-current  machine,  the  number  of 
slots  on  the  stator,  the  operating  voltage,  speed,  phase  and 
frequency  of  the  supply  circuit  are  points  that  must  be  con- 
sidered in  laying  out  a  new  winding  or  reconnecting  an  existing 
one.  The  fundamental  requirements  of  windings  and  the  ways 
in  which  they  can  be  fulfilled  as  outlined  by  M.  W.  Bartmess 
(Electric  Journal,  Vol.  VIII,  No.  5)  are  given  in  what  follows. 

Group  Windings. — Group  winding  may  be  defined  as  that 
class  wherein  the  total  winding  is  divided  into  separate  parts, 
composed  of  adjacent  coils  or  conductors.  The  grouping  is, 
in  the  case  of  lap  and  wave  windings,  an  arbitrary  one,  the 


36 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


coils  being  all  similar  and  divided  into  groups  solely  by  their 
connections.  The  number  of  coils  per  group  may  equal  the 
number  per  pair  of  poles  divided  by  the  number  of  phases,  or 
the  number  per  pole  divided  by  the  number  phases.  The  latter 
method  of  grouping  is  generally  used  on  modern  machines. 
In  the  case  of  a  six-pole,  three-phase  winding  of  36  slots,  the 
number  of  coils  per  group  is  36  -j-  (6  X  3)  or  2.  When  the 
number  of  slots  is  not  evenly  divisible  by  the  product  of  poles 


FIG.  25. — Methods  of  connecting  pole-phase  groups  shown  in  (a),  (6)  and  (c). 
(a)  Four-pole   winding  with   alternately   positive  and   negative  pole-phase  groups, 
(fc)  Four-pole  winding  of   the    consequent    pole    type,     (c)  Two-pole  winding  obtained 
from  (b)  by  reconnecting  the  pole-phase  groups  alternately  positive  and  negative. 

and  phases,  dissimilar  groups  must  be  employed.  In  such 
cases  it  is  advisable  to  arrange  the  grouping  so  that  all  the 
phases  have  an  equal  number  of  coils,  and  if  possible  the  group- 
ing should  be  arranged  symmetrically  with  respect  to  the  core 
itself.  To  prevent  local  currents,  which  may  prove  injurious, 
all  circuits  which  are  in  parallel  must  have  an  equal  number  of 
coils  and  should  be  symmetrically  arranged  with  respect  to 
each  other  and  to  the  other  phases. 

Although  one  turn  coils  only  are  shown  in  Figs.  27  to  29, 
the    same    connections    are    applicable    to    windings    having 


ALTERNATING-CURRENT  WINDINGS  37 

any  number  of  conductors  per  coil.  These  conductors  may 
all  be  in  series,  in  which  case  there  is  one  lead  at  each  end  of  the 
coil  or  the  conductors  may  be  divided  into  any  number  of  equal 
parallels,  in  which  case  there  are  as  many  leads  at  the  ends  of 
the  coils  as  there  are  parallel  circuits.  The  leads  at  the 
beginning  and  end  of  the  coils  are  connected  in  the  same  man- 
ner as  indicated  for  the  one  turn  per  coil  winding.  For  the 
sake  of  simplicity  the  number  of  coils  per  group  and  hence  the 
total  number  of  coils  in  the  diagrams  has  been  kept  lower  than 
is  generally  found  in  commercial  machines. 

Full  and  Fractional  Pitch  Windings. — The  number  of 
slots  in  the  core,  divided  by  the  number  of  poles  gives  a  value 
of  the  pole  arc  expressed  in  terms  of  the  slots.  A  full  pitch 
winding  is  one  in  which  the  effective  span  of  the  coils  is  equal 
to  the  pole  arc,  and  a  fractional  pitch  winding  is  one  in  which 
the  effective  span  of  the  coils  is  not  equal  to  the  pole  arc.  For 
a  two  coil  per  slot,  lap  or  wave  winding,  the  effective  span  of  the 
coil  is  equal  to  the  actual  span  of  the  coil.  In  this  case  the 
full  pitch  winding  is  one  where  the  coil  throw  is  equal  to 

/total  number  of  slots    ,       A       ^ 

I  —    — r e~ — r~  ~  plus  1 ) .     For  a  one  coil  per  slot  lap 

\     number  of  poles  / 

winding  the  effective  span  of  the  coil  may  be  greater  or  less  than 
its  actual  span.  In  Fig.  26  (a)  and  (6)  show  two  different  coils, 
in  each  of  which  the  effective  span  is  the  full  pitch  of  12  slots 
while  the  actual  span  in  (a)  is  only  11  slots  and  that  in  (6) 
is  13  slots.  Needless  to  say,  (a)  is  more  generally  used  on 
account  of  the  saving  in  copper  and  space  for  end  connections. 
A  coil  with  a  span  either  less  or  greater  than  that  shown 
would  result  in  a  fractional  pitch,  as  in  (c)  and  (d) . 

Representative  cases  of  concentric  group  windings  are 
shown  in  Fig.  26,  (e)  and  (f),  (e)  representing  a  three-bank 
winding,  in  which  the  number  of  coils  per  group  equals  the 
total  number  of  coils  per  phase  divided  by  the  number  of  poles, 
while  (f)  represents  a  two-bank  winding  of  the  consequent 
pole  type  in  which  the  number  of  coils  per  group  equals  the 
total  number  of  coils  per  phase  divided  by  the  number  of  pairs 
of  poles.  Neither  of  these  types  can  be  conveniently  wound 
with  a  fractional  pitch,  especially  with  formed  coils.  Where 
dissimilar  groups  are  employed,  that  is  where  the  number  of 


38 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


slots  is  not  evenly  divisible  by  the  product  of  phases  and  poles, 
the  full  pitch  is  frequently  not  a  unit  and  hence  a  fractional 
pitch  is  necessary.  In  the  case  of  a  three-phase,  four  pole 
winding  with  30  slots,  30  coils,  two  coils  per  slot,  full  pitch 
covers  a  span  of  7.5  slots  and  the  nearest  lower  even  pitch  gives 
a  throw  of  1-8. 

In  general,  fractional  pitch  affects  the  performance  of  the 
apparatus  similarly  to  a  reduced  number  of  turns  in  the  wind- 


+  +  +  +O  OOO  •  •  •  •  4 -1-4.4. 


(X)  (/) 

FIG.  26. — Possible  pitch  for  one  coil  per  slot  windings. 

(a)  Full  pitch,  effective  space  12,  actual  space  11,  throw  1  to  12.  (6)  Full  pitch, 
effective  space  12,  actual  space  13,  throw  1  to  14.  (c)  Fractional  pitch,  effective 
space  10,  actual  space  9,  throw  1  to  10.  (d)  Fractional  pitch,  effective  space  10, 
actual  space  15,  throw  1  to  16,  (e)  Concentric  group,  full  pitch,  effective  space  12, 
actual  space  9  and  11,  throw  1  to  11.  (/)  Concentric  group,  full  pitch,  consequent 
poles,  effective  space  12,  actual  space  9,  11,  13,  and  15,  throw  1  to  13. 


ing,  but  not  in  the  same  proportion.  In  a  generator  this  re- 
duces the  voltage  of  the  machine.  In  an  induction  motor,  the 
maximum  available  torque  is  increased  but  the  densities  in  the 
magnetic  circuit  are  also  increased  with  a  resulting  reduction  of 
power-factor.  For  either  motor  or  generator,  considerable 
copper  may  thus  be  saved  in  the  coil  ends  and  a  standard 
frame  may  frequently  be  used  for  special  purposes. 

Simple  Winding  Diagram. — It  is  evident  that  for  all  com- 
binations the  number  of  diagrams  necessary  would  be  unlimited. 
A  simplified  diagram  may  be  employed  which  will  not  only 
reduce  the  required  number  of  such  diagrams  but  will  also 


ALTERNATING-CURRENT  WINDINGS 


39 


minimize  the  labor  in  tracing  out  the  connections.  Thus  it  will 
be  seen  that  the  diagram  in  Fig.  27,  will  satisfy  many  require- 
ments for  connections  of  groups.  In  addition  to  this  it  will 
apply  for  any  similarly  connected  three-phase,  four-pole,  series 
star  lap-winding,  irrespective  of  the  number  of  coils  per 
group  (provided  the  groups  are  regular)  or  of  the  throw  of  the 
coils,  that  is,  whether  the  winding  is  full  or  fractional  pitch. 
This  information  for  the  throw  of  the  coils  and  the  number  of 
coils  per  group  may  be  carried 
on  the  same  specification  with 
the  remaining  winding  con- 
stants. The  groups  are  formed 
by  connecting  the  required  num- 
ber of  coils  together,  the  end  of  c 
the  first  coil  to  the  beginning  of 
the  second,  etc.,  the  beginning 
of  the  first  coil  and  the  end  of 
the  last  coil  in  the  group  forming 
the  beginning  and  end  of  the 
group.  Such  diagrams  may  be 
made  for  any  number  of  phases, 
poles  or  possible  parallel  circuits, 
and  for  any  desired  method  of 
connection  of  the  groups.  In 

case  the  coils  per  group  are  irregular  or  unbalanced,  it  is  advisa- 
ble to  have  a  special  diagram  giving  the  number  of  coils  in  each 
group,  their  location  and  any  other  information  necessary. 

Reconnecting  a  Winding. — It  is  obvious  that  if  a  winding 
gives  satisfactory  operation  on  a  certain  voltage,  a  similar 
winding  of  one-half  the  number  of  series  conductors,  but  of 
double  the  current  carrying  capacity,  will  give  satisfactory 
operation  on  one-half  the  voltage.  This  latter  condition  may 
be  obtained  by  paralleling  the  groups,  as  in  Figs.  28  and  29,  or 
where  this  is  impossible  by  paralleling  the  series  conductors  in 
the  slots.  For  example,  if  the  full  voltage  connection  of  a 
14-pole  motor  corresponds  to  the  parallel  connection  the  only 
method  to  change  to  half  voltage  would  be  to  change  the  wind- 
ing itself  since  14  poles  does  not  permit  of  a  four-parallel 
connection.  Again  an  irregularity  of  coils  per  group  will  at 


FIG.  27. — General  winding  dia- 
gram for  a  3-phase,  4-pole  motor 
with  pole-phase  groups  connected 
in  series  star. 


40 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


times  prevent  doubling  the  parallel  circuits  where  otherwise 
this  might  be  possible. 

When  it  is  desired  to  use  one  winding  for  either  full  or  half 
voltage,  the  winding,  if  possible,  is  laid  out  for  equally  satis- 
factory operation  on  either  connection,  and  for  the  minimum 
amount  of  labor  required  to  connect  from  one  to  the  other. 
This  is  exemplified  by  Figs.  28  and  29.  It  is  evident  that  any 
eccentricity  of  the  rotor  with  respect  to  the  stator  will  affect 
equally  the  circuits  which  are  in  parallel. 

Simple  Method  for  Indicating  Polarity  of  Coil  Groups. — 
A  simple  method  for  obtaining  the  proper  polarity  of  the  groups 
is  indicated  in  Figs.  27  and  28  for  three-phase  and  Fig.  29 


FIG.  28.  —  Winding  diagram  for  a 
3-phase,  4-pole  motor  with  pole- 
phase  groups  connected  in  2-parallel 
star.  This  is  the  winding  of  Fig.  27 
connected  in  parallel. 


FIG.  29. — Winding  diagram  for  a 
2-phase,  14-pole  motor  with  pole- 
phase  groups  connected  in  2  parallels. 


for  two-phase  winding.  In  a  three-phase  star  winding  by 
traveling  from  each  of  the  three  leads  to  the  star  points,  the 
direction  of  travel  is  reversed  in  adjacent  groups.  In  a  two- 
phase  diagram  the  only  necessary  precaution  in  determining 
the  proper  polarity  is  to  remember  that  adjacent  groups  of  the 
same  phase  are  reversed.  By  marking  the  groups  A,  B,  C, 
etc.,  and  indicating  the  direction  of  travel,  it  is  a  simple  matter 
to  connect  them  in  the  proper  direction.  Additional  index 
marks  may  be  given  on  the  diagram  to  aid  in  connecting  the 
winding,  by  marking  those  group  ends  which  are  joined  by  the 


ALTERNATING-CURRENT  WINDINGS  41 

same  connector,  with  the  same  numeral.  For  further  details 
in  using  this  method  see  Chapter  XI. 

Changing  Star  to  Delta  Connection. — Any  star  diagram  can 
be  readily  changed  into  a  corresponding  delta  diagram  by 
opening  up  the  star  points  and  connecting  the  inner  end  of 
phase  A  to  the  outer  end  of  phase  B  or  C,  the  inner  end 
of  phase  B  to  the  outer  end  of  phase  C  or  A,  and  the  inner  end 
of  phase  C  to  the  outer  end  of  phase  A  or  B.  If  the  star 
diagram  is  not  symmetrical  with  respect  to  the  three  phases 
it  is  never  advisable  to  change  over  to  delta. 

A.-C.  Wave  Windings. — In  a  wave  winding,  correspond- 
ingly placed  conductors  under  adjacent  poles  are  connected 
in  series,  the  circuit  proceeding  from  pole  to  pole  one  or  more 
times  around  the  core,  and  not  forward  and  back  upon  itself 
as  in  a  lap  winding.  The  circuits  are  then  interconnected  in 
such  a  manner  as  to  give  the  requisite  phase  relations.  The 
total  number  of  these  circuits  must  be  a  multiple  of  the  number 
of  phases  and  is  ordinarily  twice  the  number  of  phases.  Due  to 
certain  limitations,  this  type  of  winding  is  not  used  to  as  great 
an  extent  as  the  lap  or  the  concentric  windings.  Its  use  on 
small  motors  is  limited  to  phase-wound  secondaries.  Since  a 
two-phase  secondary  would  require  four  collector  rings,  or  if 
connected  for  a  three-wire  system,  would  overload  one  of  the 
rings,  while  a  three-phase  winding  requires  but  three  rings,  the 
latter  only  is  general  for  such  applications. 

Progressive  and  Retrogressive  A.-C.  Wave  Windings. — The 
number  of  slots  for  a  wave  winding  (plus  or  minus  one)  is 
so  chosen  as  to  be  divisible  by  the  number  of  pairs  of  poles 
or  preferably  by  the  number  of  poles.  If  plus  one,  it  is  said 
to  be  a  progressive  winding,  since  after  traveling  once  around 
the  circuit  it  returns  to  the  starting  slot  plus  one.  If  minus 
one,  it  is  said  to  be  retrogressive  since  the  circuit  returns  the 
winding  to  the  starting  slot  minus  one.  With  this  arrange- 
ment it  is  impossible  to  balance  the  phases  exactly,  but  the 
effective  unbalancing  is  small  with  a  large  number  of  slots. 
Since  an  unbalanced  three-phase  winding  is  less  objectionable 
than  a  two-phase  winding,  the  scheme  is  used  chiefly  for  the 
former.  Again,  since  an  unbalanced  winding  is  less  detri- 
mental in  a  secondary  circuit  than  in  a  primary,  the  principal 


42 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


application   of  this  type  of  winding  has  been  in  secondary 
circuits. 

The  diagram  of  Fig.  30  represents  a  two  conductor  per  slot 
winding,  such  as  a  bar  and  end  conductor  type  but  is  equally 
applicable  to  strap  or  wire  wound  coils  of  two  or  more  series 
turns  per  coil,  in  which  case  the  connector  on  the  rear  end 
of  the  coil  takes  care  of  itself  and  the  front  end  is  connected 

in  a  manner  similar  to  the 
sketch.  There  are  several 
methods  of  connecting  up  the 
three-phases  depending  on  the 
desired  voltage.  The  princi- 
pal connection  is  indicated  in 
Fig.  30.  By  connecting  the 
end  of  each  series  circuit  to 
the  beginning  of  the  next  and 
taking  off  leads  to  the  point 
of  connection  of  series  1—2, 

FIG.     30.— Wave    winding    for     a     3~4,    5-6,    instead  of  the  COn- 

3-phase,  4-poie  motor,  having  19  slots,    nections   between   the   series 

19  coils,  two  coils  per  slot,  throw  1  to       ,  •       -n-        on 

6,  with  unbalanced  phases  connected     shown   in    Fig.   30,    a    COnnec- 

in  series  star.  tion  is  obtained  for  one-half 

First  series  begins  in  bottom  slot  6,  ends     voltage.         An     86     per     cent. 
in  top  slot  12.    Second  series  begins  bot- 
tom slot  17,  ends  top  slot  4.    Third  series     Voltage     tap     in    terms    of    the 


connections  shown  in  Fig.  30 
is  secured  by  connecting  the 


begins  bottom  slot  9,   ends  top  slot   15, 

Fourth  series  begins  bottom  slot  1,  ends 

top    slot    7.      Fifth   series   begins   bottom 

slot    12     ends   top   slot   18.      Sixth  series     end    Qf    geries    2    to    the  begin- 

begins  bottom  slot  4,  ends  top  slot  1.  p  v' 

ning  of  series  3,   the  end  cf 

series  4  to  the  beginning  of  series  5,  the  end  of  series  6  to 
the  beginning  of  series  1.  To  connect  in  star  join  the  ends 
of  series  1,  series  3,  and  series  5,  and  take  off  leads  at  the 
beginning  series  6,  series  2,  and  series  4.  Since  this  connec- 
tion reduces  the  voltage  without  increasing  the  cross  section 
of  the  copper  the  winding  will  be  less  efficient  on  account  of 
higher  copper  loss  for  the  same  output.  This  is  also  true  of 
the  50  per  cent,  voltage  connection,  since  for  the  output  the 
current  density  in  the  windings  is  15  per  cent,  greater  than 
for  the  full  voltage  connection. 

It  is  possible,  by  choosing  a  number  of  slots  which  is  divisible 


ALTERNATING-CURRENT  WINDINGS 


43 


by  the  product  of  the  number  of  phases  by  the  number  of 
poles,  to  lay  out  a  winding  which  is  balanced.  For  such  a 
winding  the  circuit  after  passing  once  around  the  armature, 
returns  to  the  starting  slot.  It  is  then  only  necessary  to 
supply  a  special  connector  to  join  it  to  the  conductor  in  the 
starting  slot  plus  or  minus  one.  This  winding  thus  embodies 
the  best  features  of  both  types  as,  for  example,  an  electrical 
balance,  and  a  minimum  number  of  special  connections,  which 
means  a  very  compact  and  easily  assembled  winding.  The 
number  of  special  connections  is  comparatively  small  with 
respect  to  the  number  of  coils,  and  this  feature  is  more  pro- 
nounced as  the  number  of  poles  is  increased.  Hence  for  a 
winding  for  a  large  number  of  poles,  12  to  40,  the  number  of 
special  connections  becomes  insignificant. 

Connections  for  Coils  of  Polyphase  Windings.1 — A  two- 
phase  winding  for  a  four-pole  alternator  is  shown  in  the  sim- 
plest possible  form,  as  a 
radial  diagram,  in  Fig.  31. 
It  is  a  concentrated  wave 
winding,  having  one  slot  per 
pole  per  phase,  full  pitch. 
Each  phase  has  one  element 
of  winding,  or  one  slot,  under 
each  pole,  and  the  elements 
of  phase  B  are  distant  from 
the  corresponding  elements 
of  phase  A  by  exactly  90 
electrical  degrees.  The 


FIG.  31. — Radial  diagram  of  a  2-phase 
winding  for  a  4-pole  machine. 


whole  winding  is  thus  di- 
vided into  two  exactly  sim- 
ilar halves,  electrically  dis- 
tinct from  each  other,  just  like  two  single  phases,  their  positions 
being  relatively  fixed  so  that  Sa  passes  under  middle  Ni  just 
one-quarter  period  after  (or  before,  depending  on  direction 
of  rotation)  Sb  passes  the  same  point.  They  are  thus  tied 

1  The  following  descriptions  and  illustrations  of  windings,  pages  43  to 
51,  have  been  taken  by  permission  from  Alternating-Current  Electricity 
by  W.  H.  Timble  and  H.  H.  Higbie  (John  Wiley  &  Sons,  Inc.,  New  York, 
N.  Y.). 


44 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


UJ 


together  in  phase  relation  through  the  magnetic  field  and 
the  mechanical  distribution  of  the  winding.  The  slip  rings 
of  phase  A  would  be  connected  to  Sa  and  Fa,  and  the  rings 
of  phase  B  to  Sb  and  Fb. 

A  three-phase  winding  for  a  four-pole  machine  is  shown  in 

Figs.  32  to  34.  The  winding 
is  the  simplest  practicable  one 
to  make,  occupying  only  one 
slot  per  pole  per  phase.  This 
winding  is  laid  out  as  follows: 
First,  make  sure  that  the  total 
number  of  slots  is  divisible  by 
3  (phases)  and  4  (poles).  Then, 
holding  the  rotor  stationary, 
mark  One  element  (or  slot) 
under  the  middle  of  each  pole 

FIG.  32.— Radial  diagram  of  a    as  belonging  to  phase  A,  and 

connect  them  properly  to- 
gether in  series  so  that  their 
emfs  add  together  when  they  are  in  the  position  where  the 
maximum  instantaneous  emf  for  the  whole  group  is  induced 
(as  shown  in  Fig.  32).  Mark  one  end  of  this  series  Sa  as  in 


3-phase    winding  for  a  4-pole  ma- 
chine,  star-connected. 


FIG.  33. — Developed    diagram  of    the    3-phase    winding   of    Fig.  32,    delta- 
connected. 

Fig.  32,  and  the  other  end  Fa.  With  the  rotor  still  fixed  in 
the  same  position  and  starting  from  Sa,  proceed  to  count  the 
slots  or  coils  in  one  direction  around  the  armature  until  two- 
thirds  of  those  which  lie  between  the  middle  of  adjacent 


ALTERNATING-CURRENT  WINDINGS  45 

poles  have  been  passed.  Since  the  distance  between  the 
middle  of  adjacent  poles  is  180  electrical  degrees,  the  dis- 
tance passed  over  is  120  electrical  degrees.  Label  this  slot,  ele- 
ment or  coil  Sb  and  locate  the  other  three  B  elements  or  coils  with 
respect  to  each  other,  exactly  as  the  A  elements  are  related 
to  each  other.  Connect  the  B  group  in  additive  series  exactly 
as  the  A  group  was  connected,  and  mark  Fb  on  the  finishing 
end  of  the  B  series.  Now  from  $5  continue  to  count  slots  or 
coils  around  in  the  same  direction  until  you  have  passed  as 
far  ahead  of  Sb  as  Sb  is  ahead  of  Sa.  This  will  be  120  electri- 
cal degrees  from  S&  or  240  electrical  degrees  from  Sa.  Mark 
this  slot  or  coil  Sc?  and  locate  the  other  C  slots  or  coils  in  simi- 
lar positions  with  respect  to  all  other  poles.  Connect  the  C 
slots  together  in  additive  series  just  as  the  A  slots  were  con- 
nected, and  put  the  Fc  label  on  the  finishing  end  of  the  series. 
If  the  emf .  in  phase  A  is  at  its  maximum  value  from  Sa  to  Fa 
at  the  position  shown  in  Figs.  32  and  33,  it  is  apparent  that 
the  emf  Sa  to  Fa  must  pass  through  one-third  cycle  or  120 
electrical  degrees  before  the  emf.  from  Sb  to  Fb  reaches  its 
maximum  value  (or  is  brought  into  the  same  position  in  the 
magnetic  field  by  a  counter-clockwise  rotation  of  the  rotor). 
Also,  the  emf  from  Sa  to  Fa  must  pass  through  %  cycle  or 
240  electrical  degrees  before  the  emf  from  S.,  to  Fc  reaches  its 
maximum  value.  Since  these  three  emfs  reach  their  maxi- 
mum value  in  a  direction  away  from  S  just  120  electrical  de- 
grees apart,  consecutively,  the  S  ends  must  be  connected 
together  to  neutral,  to  get  a  star-connection.  The  terminals  of 
the  three-phase  armature  thus  connected  in  star  are  Fa,  Fb, 
Fc,  as  shown  in  Fig.  32.' 

A  developed  view  of  this  same  winding  (three-phase,  four- 
pole,  one  slot  per  pole  per  phase)  is  shown  in  Fig.  33,  connected 
in  delta  or  mesh.  Notice  that  if  only  that  end  of  each  phase 
is  marked  which  is  separated  by  120  electrical  degrees  from 
the  similar  end  of  the  preceding  phase,  the  connection  be- 
comes very  simple,  because  we  merely  connect  the  finishing 
end  of  one  phase  to  the  starting  end  of  the  next  phase  120  de- 
grees ahead,  and  so  on.  Thus  connect  Fa  to  Sb,  Fb  to  Sc  and 
Fc  to  Sa-  These  junction  points  are  then  the  terminals  of 
the  delta  winding.  The  equal  emfs  (Sa  to  Fa),  (Sb  to  Fb), 


46 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


FIG.  34. — Three-phase  winding 
having  two  slots  per  pole  per  phase. 
Two  phases  only  are  shown. 


(Sc  to  Fc)   are   120  electrical  degrees  apart  successively    in 

the    same    direction    through    the    closed    mesh    or    series. 

This  relation   gives  a  resultant  emf  of   zero   volts   around 

the  mesh. 

If  the  winding  is  to  occupy  two  slots  per  pole  per  phase, 

requiring  3  X  4  X  2  =  24  slots  altogether,  the  connections 

for  a  two-layer  lap  winding  are 
shown  in  Fig.  34.  There  are  as 
many  coils  as  slots,  and  all 
coils  are  exactly  alike.  Each 
slot  contains  two  coil-sides  (Fig. 
35).  The  one  at  the  bottom  is 
the  right-hand  side  of  a  coil 
lying  to  the  left  of  the  slot,  and 
the  one  at  the  top  is  the  left- 
hand  side  of  a  coil  lying  to  the 

ri§ht  of  the  slot'  The  elements 
at  the  bottom  of  the  slot  are 

shown  in  dotted  lines. 
Such  an  arrangement  of  coils  is  typical  'of  all  lap-wound 
direct-current  machines,  and  synchronous  converters;  but 
it  may  be  used  for  any  sort  of  an  alternating-current  winding, 
closed  or  open.  If  the  similar  end  of  every  coil  at  its  starting 
end  is  labeled  S,  and  the  other  end  F  (as  it  is  wound  up  on  a 
form,  for  instance),  then  a  closed  winding  is  obtained  by  simply 
soldering  the  F  of  one  coil  to  the  S  of  the  one  lying  in  the  next 
slot,  and  so  on  all  around  the  armature  until  the  last  F  is 
soldered  to  the  first  S.  This  closed  winding  could  be  tapped 
at  equidistant  points,  depending  on  the  number  of  poles. 
Or  this  closed  winding  could  be  opened  up  at  two  or  more 
points  and  the  parts  connected  in  series  as  an  open  winding  for 
any  number  of  phases.  The  winding  of  Fig.  35  may  be 
recognized  for  a  four-pole  stator,  because  each  coil  spans  one- 
quarter  of  the  circumference.  To  get  the  greater  emf  in  a 
coil,  its  opposite  sides  should  both  come  as  nearly  as  possible 
simultaneously  under  the  middle  of  adjacent  poles.  There  are 
24  coils  altogether,  for  three  phases  and  four  poles,  which  allows 
two  coils  per  pole  per  phase.  These  two  will,  of  course,  be  adja- 
cent coils,  in  order  that  their  emfs  shall  be  as  nearly  as  possible 


ALTERNATING-CURRENT  WINDINGS 


47 


in  phase  with  each  other  so  as  to  get  the  greatest  possible 
resultant  emf .  from  the  series. 

Double-layer  Winding,  Lap  Connected. — In  connecting  a 
winding  such  as  shown  in  Fig.  35,  choose  a  coil  located  exactly 
under  the  middle  of  NI  and  Si,  and  label  it  AI.  Mark  its 
starting  end  Sa.  Connect  the  finish  end  of  coil  AI  to  the 
starting  end  of  coil  A2  which  is  adjacent  to  coil  AI.  Now 


FIG.  35. — TWO  layer  winding  for  a  3-phase,  4-pole  machine. 


FB  S*          Fc 

L_^ 


S8 


FIG.  36. — Winding  of  Fig.  35,  delta-connected. 


FB        SA        Fc         Sa  Sc 

999          ?    Neutral  9 


FIG.  37. — Winding  of  Fig.  35,  star-connected. 

locate  coils  Az  and  A 4>  also  belonging  properly  to  phase  A, 
because  they  are  located  with  relation  to  pole  Si  and  N2  jus. 
exactly  as  coils  A\  and  A2  are  located  with  relation  to  pole 
NI  and  Si.  Similarly,  locate  A5  and  A$  under  poles  N2  and 
£2,  and  A 7  and  A8  under  poles  S2  and  NI.  Then  group  A3 
and  A 4  in  additive  series  by  soldering  the  finish  of  As  to  the 
start  of  A 4.  Similarly,  connect  A5  and  AG  together,  and  A^ 
and  AS  together.  Now,  since  the  emf  is  clockwise  around 
coils  AI  and  A2,  counter-clockwise  around  coils  A3  and  A 4, 


48  ARMATURE  WINDING  AND  MOTOR  REPAIR 

clockwise  around  A5  and  A6,  and  counter-clockwise  around 
A7  and  A8,  it  will  be  seen,  that  in  order  to  get  these  groups  of 
coils  together  into  additive  series  AI,  A2,  A5  and  A6  must  be 
connected  together  similarly  but  A3,  A4,  A7  and  A8  oppositely. 
If  the  series  of  coils  composing  phase  A  are  carefully  traced 
starting  at  Sa  and  going  right  through  to  Fa,  it  will  be  seen 
that  the  instantaneous  emfs  are  all  in  the  same  direction  at 
about  the  same  time  when  the  emfs  induced  in  the  coils  of 
phase  A  are  greatest  (which  is  about  the  position  shown  in  the 
diagram). 

Phase  B  has  been  started  at  one  end,  Sb  (similar  to  the  end 
/S0),  of  a  coil  located  120  electrical  degrees  from  coil  AI,  and 
from  this  point  through  to  Fb  the  connections  and  arrangement 
of  coils  are  an  exact  duplicate  of  phase  A,  except  as  to  actual 
position  in  the  magnetic  field.  Likewise  phase  C  is  a  duplicate 
of  phase  AI,  but  Sc  is  located  120  degrees  further  along  in  the 
same  direction  from  Sb,  or  240  degrees  from  Sa-  This  gives 
the  six  terminals  of  the  three  phases  all  properly  labeled.  In 
Fig.  36  are  shown  the  proper  connections  between  these  six 
terminals  to  give  a  three-phase  delta.  In  Fig.  37  are  shown 
the  connections  between  the  same  six  terminals  to  give  a 
three-phase  star. 

Connecting  a  Chain  Winding. — The  chain  winding  has 
been  sometimes  used  by  makers  for  alternating-current  genera- 
tors, in  addition  to  the  two-layer  windings.  Fig.  38  repre- 
sents a  three-phase  four-pole  chain  winding,  using  two  slots 
per  pole  per  phase  on  the  same  24-slot  armature  which  we  have 
been  using  throughout  for  illustration.  In  order  not  to  confuse 
the  diagram,  phases  A,  B  and  C  have  been  drawn  out  sepa- 
rately, in  Figs.  38,  39  and  40.  Notice  that  they  are  exactly 
alike,  except  as  to  relative  position  on  the  armature.  Phase 
B  is  120  electrical  degrees  from  phase  A,  and  phase  C  is  120 
electrical  degrees  further  in  the  same  direction,  from  phase  B, 
or  240  electrical  degrees  from  phase  A,  the  positive  direction 
of  emf  in  each  phase  being  from  the  S  end  to  the  F  end. 
When  the  three  phases  are  assembled  altogether  as  in  Fig.  41, 
it  is  seen  that  there  must  be  a  different  shape  or  length  of  coil 
for  each  phase  in  order  that  the  ends  of  the  coils  shall  not 
interfere  with  each  other.  This  is  expressed  usually  by  saying 


ALTERNATING-CURRENT  WINDINGS 


49 


that  the  end-bends  of  the  coils  are  in  three  ranges.     This  is  due 
to  the  fact  that  the  winding  is  a  single- layer  winding  (the 


22    23 


19  20    21  22 


FIG.  38. — Phase  A  of    a    3-phase    chain  winding.     Two  slots  per  pole  per 


FIG.  39. — Phase  B  of  the  3-phase  chain  winding  of  Fig.  38. 


21   22 


FIG.  40.— Phase  C  of  the  3-phase  chain  winding  of  Fig.  38. 


FIG.  41.— -Three-phase  chain  winding  for  which  the  separate  phases  are 
shown  in  Figs.  38,  39  and  40.  End  connections  of  phase  A  only  are  shown. 
Three  forms  of  coils  are  necessary. 

coils  of  a  two-layer  winding  are  all  exactly  alike),  and  also 
because  all  coils  in  each  phase  have  been  made  the  same  shape. 


50 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


In  Fig.  41  the  end-connections  between  coils  are  shown  only 
for  phase  A,  to  avoid  confusion.  The  system  of  connections 
would  be  exactly  the  same  as  in  other  figures  which  are 
complete 

Other  Common  Windings. — A  few  other  typical  forms  of 
windings   are   illustrated   in   Figs.   42   to   46.     Fig.   42   is   a 


FIG.  42. — Three-phase  bar  winding  (wave)  using  one  slot  per  pole  per  phase, 

star-connected. 


FIG.  43. — Skew  winding  with  all  coils  alike,  each  having  one  side  shorter 

than  the  other. 


FIG.  44. — Short-coil  winding  with  each  coil  only  two-thirds  of  the  pole 
pitch.  The  emf's  of  the  two  sides  do  not  add  to  such  good  advantage  as 
in  other  types. 

three-phase  bar  winding  (wave)  using  one  slot  per  pole  per 
phase.  It  may  be  extended  as  shown  for  any  number  of 
pairs  of  poles  and  is  drawn  star-connected.  Fig.  43  is  known 
as  a  " skew-coil"  winding;  although  there  is  only  one  coil- 
side  in  each  slot,  all  coils  are  of  the  same  shape  for  all  phases. 
Conflicts  of  the  coil-ends  are  avoided  by  making  one  side  of 


ALTERNATING-CURRENT  WINDINGS 


51 


each  coil  longer  than  the  other  side.  Fig.  44  illustrates 
what  is  called  a  " short-coil  winding"  for  a  three-phase  ma- 
chine using  two  slots  per  pole  per  phase.  By  making  the 
breadth  of  each  coil  only  %  of  the  pole  pitch,  overlapping  of 
coils  is  altogether  avoided,  and  all  coils  in  the  entire  winding 
are  exactly  alike.  The  series  emfs  composing  each  phase  are 
not  added  to  as  good  advantage  as  in  other  types  of  winding 
using  coils  nearer  full-pitch,  and,  therefore,  more  copper  would 


FIQ.  45. — Creeping  winding  in  which  coils  have  a  fractional  pitch.  Three 
coils  cover  four  poles.  The  small  dash  lines  represent  slots  left  vacant 
for  clearness. 


Mil 


1  Mil 

HIM 

^<^^r^L* 

Mill 

»'  ^                              <  K 

L             ^-^-      _* 

INN 

M  iml 

Cl^2S^-> 

Mill 
<^~^^ 

JIIM 

FIG.  46.- 


-Single-phase,    whole-coiled  winding   for    8  poles  using   3  slots 
per  pole.     Armature  has  64  slots. 


be  needed  for  the  same  capacity.  The  wave-form  is  also 
likely  to  be  more  peaked.  Fig.  45  shows  a  "  creeping  wind- 
ing" in  which  the  coils  are  of  fractional  pitch  and  the  series  of 
coils  in  each  phase  are  arranged  so  as  to  gain  or  lose  one  or  more 
poles  around  the  armature.  In  Fig.  45  three  adjacent  coils 
each  spanning  240  electrical  degrees,  together  cover  720  degrees 
or  four  poles.  Fig.  46  shows  a  single-phase  whole-coiled 
winding  for  eight  poles,  using  three  slots  per  pole,  for  an 
armature  having  altogether  64  slots. 

Easily  Remembered  Rules  for  Arrangement  of  Coils  in  an 
Induction  Motor. — The  following  are  rules  that  can  be 
easily  remembered  and  cover  conditions  frequently  encoun- 
tered in  laying  out  alternating-current  windings. 

1.  Number  of  coils  per  pole-phase-group  =  No.  slots  -f-  (No, 
poles  X  No.  phases). 


52  ARMATURE  WINDING  AND. MOTOR  REPAIR 

2.  When  the  number  of  slots  is  not  evenly  divisible  by  the  number 
of  poles  times  the  number  of  phases,   dissimilar  groups  must  be 
employed.     These  groups  should  be  arranged  so  that  all  the  phases 
have  an  equal  number  of  coils.     The  grouping  should  also  be  sym- 
metrical, with  respect  to  the  core. 

3.  A  full  pitch  winding  is  one  in  which  the  span  of  a  coil  equals 
the  number  of  slots  divided  by  the  number  of  poles.     That  is  in  a 
36  slot,  six-pole,  three-phase  winding,  36  -f-  6  =  6  slots  or  the  arc  of  the 
stator  covered  by  one  pole.     A  coil  with  a  span  of  6  slots  makes  the 
winding  a  full  pitch  winding.     When  the  coil  span  is  less  than  this, 
the  winding  is  known  as  a  fractional  pitch  winding. 

4.  If  a  particular  winding  has  given  satisfaction  on  any  particular 
voltage,  a  similar  winding  of  one-half  the  number  of  series  conductors 
but  of  double  the  current-carrying  capacity  will  also  give  satisfaction 
on  one-half  the  voltage.     This  latter  condition  can  usually  be  ob- 
tained by  paralleling  the  groups  of  coils.     Or  when  a  machine  has 
two  windings  connected  in  series,  say  for  440  volts,  they  can  be  con- 
nected in  parallel  for  220  volts. 

5.  In  a  wave  winding,  correspondingly  placed  conductors  under 
adjacent  poles  are  connected  in  series,  and  the  circuit  proceeds  from 
pole  to  pole  one  or  more  times  around  the  core.     The  circuits  are 
then   interconnected   to   give   the   requisite   phase   relations.     The 
total  number  of  these  circuits  must  be  a  multiple  of  the  number  of 
phases  and  is  ordinarily  twice  the  number  of  phases.     The  number 
of  slots  for  this  winding  (plus  or  minus  one)  should  be  divisible 
by  the  number  of  pairs  of  poles  or  preferably  by  the  number  of  poles. 

"If  plus  one,  the  winding  is  progressive,  since  after  traveling  once 
around  the  circuit  it  return  to  the  starting  slot  plus  one.  If  minus 
one,  it  is  said  to  be  retrogressive  since  the  circuit  returns  the  winding 
to  the  starting  slot  minus  one.  To  have  a  balanced  wave  winding, 
the  number  of  slots  must  be  divisible  by  the  product  of  the  number 
of  poles  and  phases.  This  winding  returns  to  the  starting  slot  after 
going  once  around  the  core  arid  special  connectors  must  be  used  to 
connect  the  finish  end  to  the  conductor  in  the  starting  slot  plus  or 
minus  one. 

Simple  Rule  for  Checking  Proper  Phase  Relationship  in 
a  Two-  or  Three-phase  Winding. — A  fundamental  considera- 
tion when  checking  the  instantaneous  flow  of  current  in  a  three- 
phase  circuit,  is  to  imagine  that  when  the  current  flows  in 
the  same  direction  in  two  legs  of  the  circuit,  it  flows  in  the 
opposite  direction  in  the  third  leg.  This  principle  can  be 


ALTERNATING-CURRENT  WINDINGS 


53 


applied  to  both  motors  and  generators.  When  this  scheme 
is  applied  to  an  alternating-current  winding  in  checking  the 
connections  of  the  coils  a  great  deal  of  experience  is  required 


A 

FIG.  47. — Simple  scheme  of  alternately  reversing  arrows  of  pole-phase 
groups  to  check  correct  phase  polarity  of  a  3-phase  winding.  It  is  supposed 
in  this  case  that  current  flows  in  the  three  leads  toward  the  star  points  of  the 
winding  which  are  indicated  thus  ( *) . 


FIG.  48. — Developed  winding  diagram  for  a  4-pole  induction  motor  showing 

series-star  connected  end  connections. 

This  winding  shows  the  groups  of  phase  coils  marked  A,  B,  C  with  arrows  pointing 
in  opposite  directions  on  adjacent  pole-phase  groups.  It  is  another  way  of  showing  the 
connections  illustrated  in  Fig.  47. 

in  order  to  make  sure  that  the  leads  of  the  three-phases  of  the 
motor  are  brought  out  at  an  electrical  angle  of  120  degrees 
apart.  This  is  usually  a  separation  of  two  poles. 


54 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


FIG.  50. — A  3-phase,  6-pole  wind- 
ing having  three  coils  per  pole-phase 
group  on  a  stator  of  54  slots.  This 
winding  can  be  connected  for  2-phase 
operation  with  an  odd  grouping  of 
coils  as  shown  in  Fig.  51. 


yf 


FIG.  51. — A  2-phase,  6-pole  wind- 
ing on  a  stator  of  54  slots.  The 
3-phase,  6-pole  winding  is  shown  in 
Fig.  50. 


FIG.  52. — A  3-phase,  6-pole  wind- 
ing showing  the  arrangement  of  odd 
groups  of  coils.  The  groups  of  two 
coils  are  located  diagonally  opposite 
each  other.  The  small  arcs  num- 
bered I  to  VI  each  span  one  pole 
containing  8  coils.  The  stator  in 
this  case  has  48  slots. 


ALTERNATING-CURRENT  WINDINGS  55 

A  simple  and  more  reliable  method  for  the  repairman  to 
use  is  shown  in  Fig.  47.  In  this  scheme  it  must  be  supposed 
that  current  flows  in  all  three  leads  of  the  star  connection  to- 
ward the  point  of  the  star  connection.  And  that  in  the  case 
of  a  delta  connection  the  current  flows  around  the  three  sides 
of  the  delta  in  the  same  direction.  Then  in  either  case  for 
a  three-phase  winding,  the  polarity  of  each  of  the  pole-phase- 
groups  will  alternate  regularly  around  the  winding  and  can 
be  indicated  by  arrows  as  in  Fig.  47.  As  shown,  for  a  three- 
phase  winding  there  will  be  three  times  as  many  pole-phase- 
groups  as  there  are  poles.  By  the  use  of  this  scheme  there  is 
no  chance  for  a  reversal  of  a  phase  to  be  passed  by  not  noticed 
when  checking  the  winding. 

In  adopting  this  scheme  for  a  two-phase  winding  it  need 
only  be  remembered  that  both  groups  of  coils  in  each  phase 
must  reverse  alternately,  that  is,  they  should  be  so  indicated 
on  the  diagram  by  arrows,  as  shown  in  Fig.  29,  page  40. 

By  marking  each  of  the  pole-phase  groups  on  a  diagram, 
A,  B,  C,  to  indicate  a  complete  phase-group,  and  placing  the 
arrows  on  each  single  group  as  shown  in  Fig.  47,  the  armature 
winder  will  have  little  trouble  in  understanding  the  diagram 
and  making  the  proper  connections.  To  make  it  still  easier, 
the  group  ends  that  are  to  be  joined  by  the  same  connector 
can  be  marked  on  the  diagram  with  the  same  number. 

The  use  of  this  method  for  regrouping  coils  of  a  three- 
phase  winding  for  operation  on  a  two-phase  circuit  and  for 
any  odd  grouping  of  coils  is  shown  in  Figs.  50  to  52. 


CHAPTER  III 

REPAIR  SHOP  METHODS  FOR  REWINDING 
DIRECT-CURRENT  ARMATURES 

Dismantling  a  D.-C.  Armature. — When  an  armature  is  to 
be  entirely  rewound,  it  must  be  stripped  and  reinsulated 
throughout.  First  remove  the  wedges  in  the  slots  when 
banding  wires  are  not  used.  If  there  are  banding  wires, 
remove  them  by  filing  in  two  parts.  When  using  a  hammer 
and  chisel  in  cutting  banding  wires  be  careful  not  to  mash  the 
armature  teeth  out  of  shape.  Now  look  over  the  connec- 
tions to  the  commutator  and  the  end  connections  of  the  coils 
to  determine  whether  a  lap  or  a  wave  winding  was  used.  In 
a  lap  winding  with  formed  coils,  the  start  and  finish  terminals 
are  connected  to  adjacent  commutator  bars  while  in  the  wave 
winding  the  terminals  of  any  coil  are  a  considerable  distance 
apart  around  the  commutator.  This  distance  is  approxi- 
mately equal  to  the  number  of  commutator  bars  divided  by 
one-half  the  number  of  poles.  Also  in  a  lap  winding  the  end 
connections  of  the  coils  at  both  ends  of  the  armature  bend 
toward  the  center  of  the  coil  or  in  the  same  direction,  while 
in  a  wave  winding  they  bend  in  opposite  directions. 

The  wave  winding  usually  has  either  two  or  four  sets  of 
brushes.  Two  sets  of  brushes  only  are  needed  regardless  of 
the  number  of  poles  in  the  machine  since  there  are  only  two 
current  paths  in  parallel  through  the  armature  winding 
(see  Fig.  13).  In  the  lap  winding  there  are  as  many  paths 
for  the  current  as  there  are  poles  in  the  machine  (see  Fig.  10) 
and  there  are  usually  as  many  sets  of  brushes  as  there  are 
poles.  Simply  because  there  are  only  two  brush  holders  on  a 
multi-polar  machine,  however,  does  not  always  mean  that  a 
wave  winding  is  used.  The  commutators  of  some  few  lap- 
wound  machines  have  internal  cross-connections  so  that  the 
commutator  bars  of  the  same  potential  are  joined  together. 

56 


REWINDING  DIRECT-CURRENT  ARMATURES  57 

This  permits  the  use  of  only  two  sets  of  brushes.  In  such  a 
case  an  extra  long  commutator  is  used  which  will  serve  as  an 
indication  of  such  connections. 

Winding  Data  Needed  for  the  Dismantled  Armature.— 
After  having  made  an  inspection  of  the  winding,  enter  the 
following  data  in  a  note  book: 

1.  Number  of  armature  slots. 

2.  Number  of  coil  sides  per  slot. 

3.  Number  of  commutator  bars — when  there  are  two  coil 
sides  per  slot,  the  number  of  bars  will  equal  the  number 
of  armature  slots  in  both  lap  and  wave  windings. 

4.  Coil  throw  in  slots. 

5.  Commutator  pitch. 

6.  Number  of  turns  per  coil  and  size  of  wire  used. 

A  convenient  chart  and  diagram  for  recording  these  data  are 
shown  in  Figs.  53  and  54  as  used  by  a  large  manufacturer.  In 
case  new  coils  must  be  ordered,  this  is  the  information  that  the 
manufacturer  needs.  It  is  a  good  plan  to  have  a  note  l^ook 
with  duplicate  printed  pages  so  that  the  record  made  on  the 
first  page  can  be  transferred  to  the  next  by  apiece  of  typewriter 
carbon  paper.  The  first  sheet  can  then  be  given  to  the 
armature  winder  and  the  copy  kept  in  the  book  as  an  office 
record  for  use  in  case  the  winder  destroys  his  copy,  for  use  in 
making  up  a  bill  for  the  job  and  for  reference  in  case  further 
repairs  should  be  later  needed  on  the  same  machine.  Such  a 
record  of  repairs  often  helps  in  the  location  of  new  trouble  in  a 
repaired  machine. 

Removing  the  Old  Coils. — The  next  operation  is  to  unsolder 
the  leads  from  the  commutator,  and  proceed  to  remove  the 
coils.  This  can  be  done  usually  by  raising  the  top  sides  of 
the  coils  for  a  distance  of  the  coil  throw,  when  the  bottom  side 
of  a  coil  can  be  reached  and  the  others  taken  out  one  after  the 
other.  In  removing  the  coils,  try  and  preserve  one  in  its 
original  shape,  to  use  as  a  guide  for  winding  new  ones.  Enter 
in  the  note  book  the  number  of  turns  per  coil,  and  with  a  wire 
gauge  determine  the  size  of  wire.  Also  note  whether  the  wire 
is  single  or  double  cotton  covered.  If  the  commutator  contains 
twice  as  many  bars  as  there  are  slots  on  the  armature,  then 


58 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


each  armature  coil  will  contain  two  coils  taped  together. 
Some  armatures,  especially  those  designed  for  low  speed,  may 
contain  three  or  even  four  times  as  many  bars  as  slots.  By 


ARMATURE  WINDING  AND  CONNECTING  DIAGRAMS 


Eig.A 


K.  W. 
H.  P.~~ 

Serial  No. 


Volts 


Xo.  Armature  Slots 


No.  Commutator  Bars 


Refer  to  Figure 


Throw  of  Coils,  A-B 


Throw 
of  Leads 


Left,  E-F. 


Total,  C-F. 
Right,  C-D 


Remarks. 


FIG.  53. — Convenient  form  for  use  in  recording  winding  data  when  new  coils 
must  be  ordered  from  the  manufacturer. 

this  construction,  the  inductance  and  the  current  per  coil  is 
kept  low,  and  the  voltage  between  bars  reduced. 

When  the  armature  has  been  stripped,  all  old  insulation 
should  be  removed  from  the  slots  by  scraping  and  burning  with 


REWINDING  DIRECT-CURRENT  ARMATURES 


59 


Electrical  Specification 
For  D.C.Xachinei 

Armature- CoU  same  asLNo 

She  of  strand  Material  Arrangement 
D.UCXRlbbon  Wide  Deep 
D.C.U.Ku.\\ire 


Method  of  Coil  Layout 


B.C.  C.  Ribbon 

D.C.C.Kd.Wlre 
D.C.C.8q.Wlre X- 

Bare  Strap 
__i Turns  per  single  coil 

_J Single  coils  per  cofl 

12  Total  Strands  per  Coil"! 

_!___  Wide  X  ___L__de«iJ 
-Silots-Two  Ooih  per  Slot 
-Com.  Bars 


i  ouptd  Ends-Strap  Commutation  Coil  Drilled  Ends 
Frame  side  JramejiJe_ 


Numbering  of  Slots  and  liars  to  bo  frju,  right  to  left 


Coil  In  bottom  of  dot 
Ko.l 


Connect  this  single 
£      Coil  to  bar  No.1 


M.      Btry,,.  j. 

tfr        "H 


Wave  winding 
progressive 
retrogressive 
with  dead  coil 
rlthout  dead  coll 


bottom  of  slot  No.l 

Connect  firet  act  he 
single  coil  on  rurht 
of  slot  No  1  to  bar 


- 1  Center  punch  marks  on  \ 
»      ends  of  com  bars         >' 


bottom  of  slot  No.1 


Bar  No. I     Bar  No. 
Bar  No.  i.   |    .  Bar  No.  Bar  No 

Bar  No. 

.....Number  of  special  coils  with  right  hand  leads  stretched  one  bar 
batween  half  idle  bmr  No and  bar  No 


Fia.  54. — Diagrams  for  recording  coil   throw  and  commutator  throw  when 

stripping  an  armature  or  when  ordering  new  coils. 

These  diagrams  are  useful  for  checking  up  a  wave  winding  that  must  duplicate  the 
original  one.  In  this  case  center  punch  marks  are  made  on  the  commutator  bars  and 
slots  to  indicate  the  coil  throw  and  commutator  connections  of  one  of  the  orignal 
coils  as  indicated  in  section  (E)  of  the  illustration.  Here  two  punch  marks  indicate 
terminal  for  coil  side  in  bottom  of  slot  and  one  punch  mark  for  the  terminal  of  the 
coil  side  in  the  top  of  the  slot.  Two  crosses  (XX)  indicates  the  slot  in  which  the  coil 
side  is  at  the  bottom  and  one  cross  (X)  the  slot  in  which  it  is  at  the  top.  When  the 
center  line  of  the  coil  is  marked  on  the  drawing  to  show  whether  it  falls  on  a  commutator 
bar  or  a  mica,  the  commutator  can  be  removed  and  replaced  to  line  up  with  the  original 
markings.  For  other  details  of  determining  the  commutator  throw,  especially  with 
dead  coils,  see  "Practical  Method  for  Locating  First  Connection  to  Commutator  for  a 
Wave  Winding"  on  page  105.  Section  (D)  of  the  above  illustration  shows  the  use  of  the 
diagram  for  marking  the  coil  throw  of  a  lap  winding.  Section  (A)  is  filled  out  to  in- 
dicate its  use  in  specifying  new  coils  for  a  wave  winding. 


60  ARMATURE  WINDING  AND  MOTOR  REPAIR 

a  torch  and  each  slot  filed  to  remove  any  burrs  or  rough  places. 
Then  clean  the  core  thoroughly  with  a  blast  of  compressed  air. 
The  core  is  now  ready  for  its  insulation  and  a  new  winding. 
Details  are  given  in  the  following  paragraphs  for  insulating 
and  winding  the  different  types  of  armatures  according  to  the 
types  of  slots  used  and  the  requirements  of  different  sizes  and 
types  of  machines. 

I.  WINDING  D.-C.  ARMATURES  HAVING  PARTIALLY  CLOSED 

SLOTS 

Direct-current  motors  of  the  industrial  type  in  sizes  of 
from  one  to  five  horsepower  are  often  built  with  partially 
closed  slots.  When  winding  the  armature  of  such  a  motor,  a 
wire-wound,  threaded-in  coil  is  used.  Practical  details  for 
rewinding  an  armature  of  this  kind  with  recommendations 
for  the  insulation  of  slots  and  testing  as  outlined  by  G.  I. 
Stadeker  in  the  Electric  Journal,  Vol.  VII,  No.  7,  are  given  in 
what  follows: 

Winding  a  Threaded-in  Coil. — Each  complete  coil  is  usually 
wound  with  from  one  to  four  separate  wires  double  cotton 
covered,  on  a  special  form  or  mould.  When  winding  the  coil 
the  two  or  more  wires  are  held  together  in  the  hand  and  wound 
as  one  wire.  Such  a  coil  is  then  made  up  of  two  or  more 
separate  coils,  and  there  will  be  as  many  beginning  ends  or 
terminals  and  finish  ends  to  the  coil  as  there  are  wires-in- 
hand  when  winding.  Each  of  these  ends  or  terminals  should 
be  provided  with  an  extra  insulation  in  the  form  of  woven 
cotton  sleeves.  As  an  aid  when  connecting  the  terminals  to 
the  commutator,  sleeves  of  different  colors  should  be  used. 
For  a  coil  wound  with  three  wires,  black,  white  and  red  sleeves 
can  be  used.  The  same  color  of  sleeve  must  be  used  on  the 
start  and  finish  ends  of  each  of  the  wires  used  in  winding  the 
coil.  To  make  sure  of  this,  two  sleeves  of  the  same  color 
should  be  slipped  over  each  wire  before  beginning  to  wind  the 
coil.  When  starting  to  wind  the  coil,  one  set  of  the  cotton 
sleeves  should  be  slipped  down  to  the  end  of  each  wire  and 
adjusted  to  reach  about  three-quarters  of  an  inch  along  the 
body  or  slot  side  of  the  coil.  The  other  set  of  cotton  sleeves 
can  be  slipped  back  along  the  wires  as  the  coil  is  being  wound. 


REWINDING  DIRECT-CURRENT  ARMATURES 


61 


Then  fasten  the  ends  of  the  wires  on  the  form  or  mould.  As 
the  spindle  on  which  the  winding  form  is  mounted  is  revolved 
slowly,  the  winder  can  guide  the  wires  into  the  mould,  placing 
strips  of  tape  under  them  at  the  corners  when  the  first  turn  is 
applied.  The  wires  should  be  kept  under  some  tension 
to  make  them  conform  closely  to  the  shape  of  the  mould  or 
form.  On  the  last  turn  another  sleeve  should  be  slipped  down 
each  wire  and  allowed  to  extend  along  the  body  of  the  coil 
a  sufficient  distance  to  be  bound  in  place  with  the  tape  which 
was  previously  inserted.  The  leads  can  then  be  cut  off  to  the 
proper  length. 

In  case  a  coil  of  a  large  number  of  turns  is  to  be  wound  and 
the  cotton  sleeves  used  do  not  slip  easily  over  the  wires,  it  may 
l>e  quicker  to  use  a  test  lamp  to  find  the  correct  finish  end  on 
which  to  use  the  proper  color  of  sleeve.  In  such  a  case  one  of 
the  start  ends  of  the  coil  can  be  placed  on  one  terminal  of  the 
test  lamp  and  each  of  the  other  finish  ends  tried  until  the  lamp 
will  light.  On  the  two  ends  thus  located  the  same  color  of 
sleeve  should  be  used.  The  second  pair  of  ends  can  be  located 
in  the  same  way. 

Insulating  Lining  for  Slots.  —  The  slots  should  be  insulated 
with  an  outer  protective  layer  of  fish  paper  (for  thickness  see 
pages  163  to  172,  on  ''Insulation  for 
Slots")  about  three-quarters  of  an  inch 
longer  than  the  slot  and  two  inner  cells 
of  treated  cloth.  One  of  these  cells 
should  enclose  the  lower  and  the  other 
the  upper  coil  as  shown  in  Fig.  55. 
For  machines  having  a  terminal  vol- 

£    r/\f\         ij.  "j.   •  j 

tage  of  500  volts  or  over  it  is  good 
practice  to  use  a  third  cell  of  treated 
cloth  placed  next  to  the  fish  paper  cell  and  enclosing  both  of 
the  coils.  This  provides  a  better  insulation  of  the  winding 
from  the  core.  These  cells  should  be  cut  so  that  they  will 
project  about  an  inch  beyond  the  slot  opening  to  serve  as  a 
guide  when  the  strands  of  the  coil  are  being  inserted  in  the  slot. 
Inserting  Coils  in  the  Slots.  —  Since  the  slots  are  partially 
closed,  all  the  bottom  sides  of  the  coils  can  be  inserted  in  the 
slots  before  the  top  sides  are  inserted.  In  each  case  the  indi- 


FIG.  55.—  Slot  insulation 

for  a  double  layer  winding 

in  partially  cl  Jed  slots. 


62  ARMATURE  WINDING  AND  MOTOR  REPAIR 

vidual  strands  may  be  forced  into  the  slot  with  a  flat  fiber 
drift.  As  each  coil  is  put  in  place  the  lower  leads  should  be 
inserted  into  the  slits  of  the  proper  commutator  bars,  care 
being  taken  that  the  different  colored  leads  are  connected 
always  in  the  same  order.  (For  details  of  connections  for 
lap  and  wave  windings,  see  Chapter  " Making  Connections 
to  the  Commutator,"  page  101). 

After  all  the  coils  are  in  position  in  the  bottoms  of  the  slots, 
the  protecting  edges  of  the  inner  cell  enclosing  the  lower 
coil  side  should  be  drawn  up  as  far  as  the  coil  side  will  allow, 
then  cut  off  close  to  the  slot  and  folded  in.  With  a  fiber  drift 
and  mallet  force  the  wires  and  cell  into  proper  position  in 
the  lower  half  of  the  slot  in  order  to  make  room  for  the 
upper  coil  side.  The  unprotected  wires  of  the  coil  which  cross 
the  end  of  the  core  should  now  be  taped  up  with  cotton  tape 
for  about  two-thirds  of  their  length,  starting  up  close  to  the 
core  and  enclosing  the  ends  of  the  lower  cells,  which  project 
from  the  slot. 


Steel  driving 
1  Slot  wedge  slide 

FIG.  56. — Device  for  driving  wood  and  fiber  wedges  in  slots. 


When  ready  to  insert  the  top  sides  of  the  coils,  the  upper 
cells  of  treated  cloth  should  be  inserted  in  the  slots  preferably 
one  at  a  time.  Now  face  the  commutator  and  count  the 
throw  already  calculated  for  the  winding  in  a  counter-clockwise 
direction  from  the  first  slot  prepared  with  its  insulating  cell, 
in  order  to  determine  which  coil  should  go  into  this  slot.  The 
proper  coil  should  then  be  bent  into  shape,  its  strands  waxed 
and  inserted  into  the  slot.  The  edges  of  the  projecting  cell 
can  now  be  clipped  and  folded  in  and  hammered  tightly  into 
place  with  the  mallet  and  drift.  Then  drive  in  a  fiber  wedge 
over  the  coil  by  using  a  wedge  driver  such  as  shown  in  Fig.  56. 
This  consists  of  a  hollow  rectangular  piece  of  steel  fitted  with  a 
sliding  steel  strip  about  the  same  size  as  the  wedge.  The  wedge 


REWINDING  DIRECT-CURRENT  ARMATURES  63 

is  inserted  into  the  driver,  its  end  beveled  and  forced  into  the 
slot  by  tapping  on  the  steel  strip  of  the  driver  with  a  mallet. 

Insulating  Overlapping  End  Connections  of  Coils. — After 
the  wedge  has  been  driven  in,  take  a  piece  of  cotton  tape  and 
wrap  firmly  around  the  coil  and  the  projecting  tip  of  the  wedge 
close  up  against  the  core  and  continue  to  tape  up  the  end  con- 
nection of  the  coil  to  the  point  where  it  was  insulated  from 
the  bottom  upward  when  the  other  side  was  placed  in  the 
bottom  of  the  slot.  Glue  the  ends  of  the  tape  where  they 
meet. 

When  two  or  three  of  the  top  sides  of  coils  have  been  placed 
in  position,  insert  one  or  more  strips  of  treated  duck 
between  the  end  connections  of  the  upper  and  lower  coils 
where  they  cross  each  other  at  each  end  of  the  armature. 
As  the  remaining  upper  sides  of  the  coils  are  placed  in  the 
slots,  these  strips  should  be  wound  around  the  armature 
so  that  they  finally  form  a  complete  band  of  insulation  between 
the  upper  and  lower  coils.  Each  coil  as  it  is  finally  placed 
should  be  shaped  at  its  ends  with  a  mallet  and  fiber  drift  so 
that  one  coil  fits  .snugly  against  the  other  and  there  is  a 
rigid  construction  when  the  armature  is  completed. 

Connecting  Finish  Ends  of  Coils  to  Commutator. — The 
ends  of  the  upper  sides  of  the  coils  can  now  be  connected  to 
the  commutator  using  a  test  lamp.  Connect  the  colored  leads 
in  proper  order  as  explained  on  page  101  under  "  Making 
Connections  to  the  Commutator."  Before  soldering  the 
coil  ends,  the  entire  winding  must  be  tested  for  grounds 
with  proper  voltage  and  for  short  circuits  and  open  circuits. 
(See  page  122  under  the  heading  of  "  Testing  Armature  Wind- 
ings.") If  no  faults  are  discovered  the  ends  of  the  coils  can  be 
soldered  to  the  commutator  and  the  latter  turned  and  finished 
with  sandpaper.  The  armature  is  now  ready  for  banding. 

Loop  Windings  for  Small  Motors. — Small  direct-current 
armatures  for  fan  motors  and  general  utility  use,  are  often 
wound  by  hand  using  what  is  sometimes  called  a  loop  winding. 
In  the  case  of  a  2-pole  armature  having  15  slots  and  15  com- 
mutator bars,  such  a  winding  might  be  made  up  as  follows: 
Wind  slots  Nos.  1  and  8  half  full,  using  double  cotton-covered 
wire,  leaving  the  ends  a  little  longer  then  needed  to  connect  to 


64  ARMATURE  WINDING  AND  MOTOR  REPAIR 


FIG.  57. — Winding  the  first  coil  in  the  slots  of  a  small  direct-current  motor. 


FIG.  58. — The  operator  is  here  shown  completing  the  last  turn  of  the  winding. 


FIG.  59. — In  this  illustration  the  operator  iri  driving  down  the  coils  before 
inserting  fiber  wedges  in  the  slots  of  the  completely  wound  armature. 
In  Figs.  57  to  59  three  steps  are  shown  in  \vinding  a  small  direct-current  armature. 
Before  starting  to  wind  the  coils,  the  cores  are  insulated  on  the  ends  as  shown  in  the 
lower  right-hand  corner  of  Fig.  57.     The  armature  illustrated  is  for  a  32-volt  motor  and 
has  19  slots  with  a  coil  pitch  of  1  to  8  slots.     Ten  turns  of  No.  11  wire  per  coil  are  used 
with  two  coils  per  slot.     There  are  a  total  of  38  coils  and  38  commutator  bars  (Robbing  & 
Myers  Company). 


REWINDING  DIRECT-CURRENT  ARMATURES 


65 


the  commutator  and  without  cutting  the  wire  wind  a  similar 
coil  in  slots  2  and  9  and  bring  out  a  second  loop.  Proceed 
in  this  way  throughout  all  the  pairs  of  nearly  opposite  slots 
until  on  the  second  round  the  slots  are  completely  filled  when 
the  beginning  and  the  end  of  the  wire  can  be  twisted  together 
for  the  15th  loop.  Cotton  sleeves  can  now  be  inserted  over 
the  loops  and  connections  made  to  the  commutator.  Figs. 
57  to  59  show  an  armature  being  wound  by  this  method. 
In  the  illustrations  the  necessary  insulation  on  the  shaft  close 
to  the  core  and  on  the  core  at  the  ends  of  the  slots  is  shown. 
The  slots  should  be  insulated  in  this  case  with  heavy  fish 
paper  about  10  mils  thick. 

Banding   a    Small   D.-C.    Armature. — When  applying  the 
banding  wire,  the  armature  should  be  inserted  in  a  lathe. 


FIG.  60. — Dissembled  view  of  a  small  direct-current  motor  showing  banded 
armature  (Fidelity  Electric  Company). 

The  first  operation  is  to  hammer  down  the  ends  of  the  coils 
until  their  diameter  at  the  armature  ends  is  no  greater  than 
that  of  the  core.  Extreme  care  must  be  taken  not  to  injure 
the  insulation  with  the  mallet.  As  a  base  for  the  banding 
wires,  two  bands  of  cotton  tape  separated  by  a  band  of  var- 
nished paper  should  be  wound  over  the  end  connections  near 
the  core  and  the  whole  tied  down  with  several  layers  of  twine. 
Short  strips  of  tinned  copper  about  0.02  by  0.25  inch  in  cross- 


66  ARMATURE  WINDING  AND  MOTOR  REPAIR 

section  should  be  slipped  under  the  temporary  banding  twine 
at  intervals  of  two  or  three  inches  with  two  extra  ones  used 
where  the  banding  wire  is  started  and  ended.  For  the  banding 
wire  of  small  armatures  No.  14  to  No.  17  B.  &  S.  gauge  tinned 
steel  wire  can  be  used.  The  start  should  be  fastened  to  a  peg 
slipped  into  an  air  duct  or  fastened  to  the  end  of  the  banding 
twine  and  the  first  layer  guided  so  that  it  crosses  itself  to 
relieve  the  strain  on  the  end  fastening.  After  two  or  three 
revolutions  of  the  band  wire  the  temporary  banding  twine 
can  be  cut  off  and  the  banding  wire  wound  on  tightly  across 
the  protecting  tape.  After  the  required  width  has  been 
wound,  the  copper  strips  at  the  start  end  should  be  turned 
up  and  clipped  off  so  that  about  one-quarter  inch  projects 
from  under  the  banding  wire.  Then  bend  the  strips  over  to 
hold  the  banding  wire  in  position. 

Without  cutting  the  banding  wire,  it  should  be  guided  across 
the  core  to  the  opposite  end  of  the  armature  and  wound  in  the 
same  manner  as  before  to  within  about  a  quarter  of  an  inch 
of  the  edges  of  the  end  connections.  The  clips  at  beginning 
and  end  can  now  be  bent  over  and  soldered.  The  banding 
wires  on  both  ends  of  the  armature  should  now  be  driven  up 
close  together,  all  the  clips  bent  over  and  the  wires  soldered 
together.  In  the  soldering  of  band  wires  no  acid  should  be 
used.  A  solution  of  rosin  in  alcohol  is  recommended.  After 
the  soldering  operation  is  completed  the  surplus  turns  and 
crossovers  of  the  banding  wire  can  be  cut  off.  The  armature 
is  now  completed  except  for  the  balancing.  (See  page  150 
under  the  heading  of  "Balancing  an  Armature.7') 

II.  WINDING  D.-C.  ARMATURES  HAVING  OPEN  SLOTS 

Motors  in  sizes  above  five  horsepower  that  nave  armature 
cores  with  open  slots  may  be  wound  with  coils  made  up  of  wire 
or  of  copper  strap.  The  coils  may  be  of  the  form  wound  or 
pull  in  types  but  are  fully  insulated  before  being  inserted  into 
the  slots.  In  general  two  types  of  open  slots  are  found,  one 
with  wedge  grooves  in  the  top  of  the  slot  in  which  wedges  can 
be  driven  to  hold  the  coils  in  position;  while  the  other  is  a  slot 
with  smooth  sides  requiring  banding  wire  to  hold  the  coils  in 


REWINDING  DIRECT-CURRENT  ARMATURES 


67 


place.  Grooves  are  usually  provided  on  the  armature  for 
bands  in  addition  to  those  over  the  end '  connections.  Very 
often  the  same  armature  core  with  open  slots  is  used  for  differ- 
ent types  and  sizes  of  machines.  The  coils  may  therefore  be 
smaller  in  some  cases  than  the  original  ones  and  do  not  fill  the 
slots.  In  these  cases  fillers  must  be  used  to  fill  out  the  sides 
and  below  the  coils  in  the  slot  so  that  the  wedges  or  band  wire 


FIG.  61. — Direct-current    armature    showing  duck  or  canvas  pad  held  by 

cordage  on  ring  which  supports  coils 

This  insulation  is  used  to  prevent  coils  from  rubbing  on  the  frame  and  gives  a  larger 
margin  of  safety  against  grounds.  Slot  insulation  made  of  treated  pressbpard  is  shown 
in  place.  Separators  between  coils  in  each  slot  are  made  of  the  same  material  but  a  little 
thicker  (Roth  Brothers  &  Company). 

will  press  firmly  on  the  top  of  the  coils  and  prevent  any  pos- 
sible motion  of  the  coils  in  the  slots  that  will  result  in  chafing 
and  damage  to  the  insulation.  Fillers  most  used  consist  of 
strips  of  treated  fullerboard  or  treated  wood.  If  a  coil  is 
only  slightly  loose  it  is  better  to  add  insulation  to  it. 

Winding  and  Insulating  Coils. — In  case  coils  must  be  wound 
for  a  particular  job  proceed  as  outlined  under  the  heading 


68  ARMATURE  WINDING  AND  MOTOR  REPAIR 

of  " Forms  for  Winding  Coils,"  on  page  141.  In  rewinding 
a  bar  wound  armatAire,  it  will  seldom  be  necessary  to  form  new 
coils,  because  each  coil  consists  of  a  single  turn  of  heavy  copper 
strip,  which  is  not  as  easily  damaged  as  are  wire  wound  coils. 
In  this  case,  all  that  will  generally  be  necessary  is  to  reinsulate 
the  coils  and  the  armature  core.  When  all  the  coils  have  been 
wound,  they  should  be  covered  with  cotton  tape  about  three- 


FIQ.  62. — Armature  shown  in  Fig.  61  with  a  partial  lap  winding  in  place. 

The  insulation  on  ends  of  coils  and  method  of  interlacing  coils  so  that  they  occupy 
a  minimum  amount  of  space  are  shown.  Coil  ends  are  later  to  be  fastened  to  supporting 
spider  ring  with  treated  canvas  or  duck  and  cordage.  Band  wires  are  wound  on  core 
with  stripes  or  pads  of  thin  canvas  underneath,  after  winding  is  completed  (Roth  Brothers 
&  Company). 

quarter  inch  in  width.  The  end  of  the  tape  winding  can  be 
made  fast  with  a  piece  of  thread,  care  being  taken  to  have  the 
taped  end  on  a  part  of  the  coil  that  does  not  go  into  the  slots, 
as  the  thickness  of  the  thread  on  each  side  may  be  sufficient 
to  cause  the  coil  to  stick  while  being  placed  in  position  on  the 
core.  The  finished  coils  should  go  into  the  insulated  slots 
without  undue  driving.  The  coils  should  now  be  given  a  coat 


REWINDING  DIRECT-CURRENT  ARMATURES          69 

of  moisture  repelling  varnish  and  allowed  to  dry.  When  thor- 
oughly dry  the  armature  can  be  rewound. 

Insulating  Open  Slots. — When  the  armature  is  not  too  large 
it  can  be  wound  in  a  lathe  or  bench  stand.  For  large  armatures 
a  suitable  floor  stand  can  be  easily  made.  Place  the  armature 
in  the  lathe  or  on  the  stand  with  the  commutator  at  the 
winder's  right.  Before  inserting  the  insulation  for  the  slots, 
they  should  be  thoroughly  cleaned  and  all  burrs  and  sharp  edges 
removed  with  a  file.  The  same  arrangement  of  slot  insulation 
described  for  partially  closed  slots  on  page  61  can  be  employed. 
However,  for  low-voltage  machines  (not  over  250  volts)  to 
be  used  in  dry  places  a  satisfactory  slot  insulation  consists  of 
two  layers  of  fish  paper  each  0.005  inch  thick  between  which  is 
placed  a  layer  of  empire  cloth  0.010  inch  thick.  This  insula- 
tion should  be  cut  so  that  it  will  extend  about  J^j  inch  past  the 
end  of  the  slot  and  project  about  one  inch  above  the  entrance 
bo  the  slots.  This  will  form  a  mechanical  protection  for  the 
coils  and  serve  as  guides  through  which  the  coils  can  be  slid 
into  place.  The  corners  of  the  projecting  edges  should 
be  clipped  to  keep  them  from  interfering  with  the  insertion  of 
the  coils.  If  end-ring  insulation  is  to  be  used,  hold  it  in  place 
by  winding  thread  over  the  ends  of  the  core  through  the  slots. 

Inserting  Coils  in  Open  Slots. — After  the  slots  have  been 
insulated,  place  one  side  of  a  coil  in  a  slot  and  force  it  to  the 
bottom  with  a  fiber  drift  a  little  narrower  than  the  width  of 
the  slot.  The  other  half  of  the  coil  (or  top  side)  should  be  left 
out  of  the  armature  for  the  present.  Insert  the  bottom  half 
of  coil  No.  2  in  the  next  slot. turning  the  armature  in  a  counter- 
clockwise direction  (looking  toward  the  commutator  end). 
Proceed  in  this  manner  until  the  top  side  of  one  coil  is  to  be 
placed  into  the  slot  containing  the  bottom  side  of  the  first 
coil  placed  on  the  armature.  This  will  be  the  first  coil  in 
which  the  top  side  can  be  placed  in  a  slot  over  the  bottom  side 
of  coil  No.  1.  Before  doing  this,  reference  should  be  made  to 
the  note  book  data  in  case  an  armature  is  being  rewound 
exactly  as  before,  in  order  to  make  sure  that  the  proper  number 
of  slots  have  been  spanned. 

To  make  this  method  of  winding  clear,  refer  to  Fig.  63. 
Here  the  coils  span  five  slots,  that  is,  the  top  side  of  coil  No.  1 


70  ARMATURE  WINDING  AND  MOTOR  REPAIR 

is  removed  five  slots  from  the  bottom  side  of  the  same  coil. 
The  top  half  of  coils  Nos.  1,  2,  3,  4,  and  5  (called  the  throw 
coils  because  they  cover  a  part  of  the  armature  equal  to  the 
throw  of  a  coil)  are  left  out  of  the  slots  as  shown,  as  the  bottom 
half  of  other  coils  will  have  to  be  placed  in  these  slots  before 
these  top  halves  can  be  put  in.  When  the  bottom  half  of  coil 
No.  6  is  placed  in  slot  11,  its  top  side  may  be  placed  in  slot  6, 
because  the  bottom  side  of  coil  1  is  located  in  this  slot.  Con- 
tinue to  place  the  coils  on  the  core,  traveling  in  the  direction 
indicated  by  the  arrow.  The  bottom  half  of  coil  No.  7  is 


FIG.  63. — Method  of  placing  throw  coils  on  the  armature  of  a  lap  winding. 

placed  in  slot  12,  and  the  top  half  in  slot  7.  Before  inserting 
the  coils  in  the  slots  it  is  a  good  plan  to  rub  the  sides  with 
paraffin  wax.  This  helps  in  inserting  the  coils  and  prevents 
damaging  the  insulation. 

As  the  armature  is  being  wound,  a  strip  of  heavy  pressboard 
(about  0.050  inch  thick)  should  be  placed  in  the  slots  between 
coils  to  thoroughly  insulate  them  from  each  other.  When  all 
the  coils  have  been  inserted,  the  top  sides  of  coils  Nos.  1,  2, 
3,  4,  and  5  can  be  placed  in  the  slots.  As  each  coil  is  put  in 
position,  the  end  connections  should  be  carefully  shaped  to 
the  core. 

Shaping  End  Connections. — The  end  connections  are  shaped 
by  means  of  a  winding  drift.  This  consists  of  a  steel  bar  about 
12  inches  long,  one  inch  wide  and  tapered  in  thickness  from 
one-half  to  one-eighth  inch,  having  all  the  corners  rounded 


REWINDING  DIRECT-CURRENT  ARMATURES 


71 


and  smooth.  The  tip  of  this  drift  should  be  placed  against  the 
inner  side  of  the  end  connection  of  the  coil  and  tapped  with  a 
mallet,  forcing  the  upper  part  of  the  end  connection  out  from 
the  armature  and  away  from  the  lower  half.  Each  coil  as  it 


FIG.  64. — Rear  and  commutator  ends  of  a  wave-wound  armature  using 
strap  coils.  Note  the  insulation  of  slots  and  the  insulated  support  of  rear 
end  connections  (General  Electric  Company). 

is  put  in  place  can  be  similarly  shaped  so  that  when  the  arma- 
ture is  completely  wound  a  circular  air  chamber  is  formed 
between  the  upper  and  lower  halves  of  the  end  connections  at 
both  front  and  rear.  This  process  should  be  continued  until 
the  first  slot  is  again  reached. 


72  ARMATURE  WINDING  AND  MOTOR  REPAIR 

Six  Steps  in  Winding  a  Small  Direct-current  Armature 

No.  1 


FIG.  65. — This  illustration  and  those  of  Figs.  66  to  71  show  successive 
steps  in  winding  a  small  direct-current  armature  (Crocker-Wheeler  Company). 
The  slot  and  shaft  insulation  and  first  two  coils  are  shown  in  place  here. 


No.  2 


FIG.  66. — Appearance  of  the  winding  before  the  last  three  coils  are  inserted  in 
bottoms  of  the  slots. 


REWINDING  DIRECT-CURRENT  ARMATURES  73 


No.  3 


iG.  67. — All  bottom  sides  of  coils  in  place  and  strips  of  insulation  inserted 
between  bottom  and  top  coil  sides  ready  for  the  top  layer  of  the  winding. 


No.  4 


FIG.  68. — Inserting  the  last  few  top  sides  of  coils.  Note  the  shaping  of  the 
end  connections  of  this  winding  alternately  in  and  out  to  give  the  compact 
appearance  shown  in  Fig.  71. 


74  ARMATURE  WINDING  AND  MOTOR  REPAIR 


No.  5 


Fiu.  G9. — Connecting  the  first  two  leads  of  bottom  coil  sides  to  the  commu- 
tator. 


No.  6 


FIG.  70. — Connecting  the  leads  of  top  coil  sides  to  the  commutator, 
the  tape  insulation  between  coil  terminals. 


Note 


REWINDING  DIRECT-CURRENT  ARMATURES 


75 


FIG.  71. — Completed  armature  banded  and  treated  with  insulating  com- 
pound.    Note  winding  insulation  over  coil  terminals  at  commutator  end. 


FIG.  72. — Portable  floor  stand  for  winding  small  direct-current  armatures- 
The  operator  is  shown  shaping  the  end  connections  with  a  fiber  drift. 


76 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


Truing  Up  the  Heads  of  the  Winding. — When  the  winding 
is  completed,  bend  the  leads  back  over  the  surface  of  the  core 
so  as  to  expose  the  head  of  the  winding.  This  must  be  trued 
by  revolving  the  armature  and  marking  the  high  places  with 
chalk.  These  high  spots  can  be  driven  down  with  a  rawhide 
mallet,  and  the  low  places  raised  even  with  the  others.  The 
back  head  should  also  be  carefully  trued  up  so  as  to  present  a 
good  appearance  when  running.  Trim  off  the  projecting 
slot  insulation  even  with  the  surface  of  the  armature  if  banding 
wires  are  to  be  used.  In  case  the  coils  are  to  be  held  in  place 
by .  wood  or  fiber  retaining  wedges,  lap  this  insulation  down 
over  the  coils  and  drive  in  the  wedges.  If  the  armature  leads 
are  not  covered  with  tape,  they  must  be  protected  in  some 
manner  and  for  this  cotton  sleeving  can  be  used. 


FIG.  73. — Insulation  of  end  connections  showing  friction  cloth  blanket  i 
place  at  the  commutator  end  as  a  protection  for  the  leads. 

Insulation    Between    Commutator    End    Connections. — A 

friction  cloth  blanket  should  be  placed  over  the  end  connec- 
tions on  the  commutator  end,  as  shown  in  Fig.  73,  as  a 
protection  for  the  terminals  of  the  coils.  It  is  good  practice 
to  shape  the  ends  or  terminals  of  the  side  of  the  coils  in  the  bot- 
toms of  the  slots  as  shown  in  this  illustration  and  to  bend 
back  over  the  core  the  terminals  of  the  top  sides  of  the  coils. 
This  helps  in  arranging  the  connections  from  the  coils  in  the 
bottoms  of  the  slots,  which  can  now  be  made. 


REWINDING  DIRECT-CURRENT  ARMATURES  77 

Now  wrap  several  thicknesses  of  treated  cloth  in  a  belt 
over  the  bottom  leads  and  bind  with  thread.  The  top  lead, 
located  by  a  lamp  tester  of  magneto  can  now  be  connected 
to  the  commutator.  In  the  same  manner  connect  the  remain- 
ing leads.  Cut  off  the  ends  of  the  leads  that  project  past 
the  neck  of  the  commutator,  and  if  there  is  space  after  the  lead 
have  been  driven  down,  these  pieces  can  be  used  as  "  dummies" 
to  fill  such  spaces.  When  this  stage  of  the  winding  process 
has  been  reached,  a  short-circuit  and  open-circuit  test  must 
be  made.  (See  page  122  under  heading  of  " Testing  D.-C. 
Armature  Windings.")  If  the  armature  tests  clear,  the  leads 
can  be  soldered  in. 

When  the  wedges  are  not  used  to  hold  the  coils  in  place  some 
means  must  be  provided  to  prevent  them  from  becoming 
loosened  from  the  slots,  during  the  subsequent  operations 
before  banding.  To  accomplish  this  a  single  band  of  wire 
can  be  tightly  fastened  around  the  coils  at  each  end  of  the 
armature. 

After  these  operations  the  commutator  can  be  turned  and 
polished  and  the  armature  banded  if  this  is  required.  When 
balanced  and  painted  with  insulating  varnish,  the  armature 
is  ready  for  use. 

m.  WINDING  LARGE  D.-C.  ARMATURES 

In  contrast  with  the  winding  of  small  armatures,  the  winding 
of  large  direct-current  armatures  is  not  a  particularly  compli- 
cated operation.  Although  the  insulation  throughout  must  be 
moisture  and  oil  proof,  no  such  elaborate  precautions  as  in 
the  case  of  industrial  motors  is  necessary  in  the  larger  sizes 
of  machines  for  they  are  in  most  cases  installed  in  dry  clean 
places.  The  centrifugal  strains,  however,  may  be  higher  in 
the  larger  machines  and  the  windings  must  also  be  braced 
against  the  magnetic  strains  produced  by  heavy  short  circuits 
which  may  occur  on  account  of  the  very  low  resistance  of 
circuits  using  wire  of  large  cross-section.  The  following 
recommendations  are  given  for  the  winding  of  large  armatures 
by  a  writer  in  the  Electric  Journal,  Vol.  VII,  No.  11. 

Coils  for  Large  D.-C.  Armatures. — For  the  large  sizes 
of  direct-current  machines  the  armature  coils  are  usually 


78  ARMATURE  WINDING  AND  MOTOR  REPAIR 

formed  of  bare  copper  strap.  They  are  usually  of  the  one 
piece  or  two  piece,  one  turn,  diamond  type  with  the  number 
of  coils  equal  to  the  number  of  commutator  bars.  To  secure 
best  possible  space  factor,  and  for  other  mechanical  reasons, 
single  coils  are  often  bound  together  into  a  larger  coil,  each 
single  coil  being  insulated  from  the  other  and  electrically 
separated.  In  such  a  case  the  number  of  slots  is  only  a 
fraction  of  the  number  of  commutator  bars. 


FIG.  74. — Lap-wound  armature  using  two-part  strap  coils.  Note  the 
four  layers  of  insulation  between  end  connections  at  commutator  end  (General 
Electric  Company) . 

The  method  of  insulating  this  type  of  coil  depends  upon 
the  size,  voltage  and  operating  conditions  of  the  machine  and 
on  the  number  of  single  coils  composing  a  complete  coil. 
When  there  are  less  than  four  single  coils  per  complete  coil, 
the  ends  of  each  single  coil  are  taped  with  one  layer  of  cotton 
tape,  half  overlapped.  This  taping  extends  a  sufficient  dis- 
tance along  the  straight  part  to  assure  that  the  joint  between 
it  and  the  rest  of  the  insulation  will  be  well  protected.  The 
straight  parts  are  then  wrapped  with  a  fish  paper  and  mica 
wrapper,  interwoven  between  the  straps  in  such  a  manner  as  to 
furnish  insulation  between  the  single  coils,  and  then  wrapped 
several  times  around  the  complete  coil,  the  exact  number  of 


REWINDING  DIRECT-CURRENT  ARMATURES  79 

turns  depending  on  the  size,  voltage  and  operating  conditions 
of  the  machine.  (See  "Coil  Insulation,"  pages  163  to  172.) 

When  there  are  four  or  more  single  coils  per  complete 
coil,  alternate  single  coils  should  be  wrapped  with  one  turn 
of  fish  paper  and  mica,  held  in  place  by  a  non-overlapping  layer 
of  cotton  tape.  The  single  coils  are  then  assembled  and  a 
cell  of  fish  paper  and  mica  wrapped  over  the  whole.  The  coil 
should  then  be  taped  with  a  layer  of  cotton  tape,  non-over- 
lapping over  the  wrapper  and  half  lapped  over  the  ends. 
Then  brush  with  or  dip  in  a  black  finishing  varnish  and  air 
dry.  After  this  dip  twice  in  insulating  varnish  and  dry  in 
an  oven  for  twelve  hours  after  each  immersion.  Before 
the  coil  is  used  in  the  armature,  the  leads  should  be  cleaned 
of  insulation  and  varnished  and  thoroughly  tinned. 

Lap  and  Wave  Windings  for  Large  Armatures. — In  the 
main  the  same  conditions  outlined  on  page  112  under  the 
heading  "Wave  Lap  vs.  Direct-current  Windings"  apply  in 
the  case  of  largo  armatures.  The  wave  winding  has  the 
decided  advantage,  however,  that  no  cross-connections  are 
required.  It  has,  therefore,  a  wide  use  in  machines  where  the 
size  of  coil  required  does  not  become  excessive  nor  the  voltage 
between  commutator  segments  too  great  to  permit  of  good 
commutation.  Under  ordinary  conditions  this  consideration 
limits  the  wave  winding  to  four-  or  six-pole  machines.  Where 
the  number  of  poles  is  greater  the  lap  winding  seems  to  suit 
the  conditions  of  good  operation  best.  With  this  winding 
the  voltage  between  segments  is  kept  down  and  high  conduc- 
tivity through  the  armature  is  made  possible  without  using 
coils  of  large  cross-sections.  For  both  lap  and  wave  windings, 
one-piece  coils  can  be  used,  but  a  two  piece  or  half  coil  has 
some  advantages  in  that  less  skill  is  needed  to  wind  the 
armature  and  the  coils  are  easily  repaired  if  the  damage  is  only 
to  the  top  half.  The  half  coil  however  calls  for  a  soldered 
joint  at  the  rear  of  the  armature  which  must  be  made  with 
great  care. 

Insulating  the  Core. — Before  starting  the  winding  operation 
the  core  should  be  thoroughly  cleaned  with  an  air  blast,  thus 
removing  any  iron  filings  or  other  foreign  matter  from  the 
slots.  The  commutator  necks  must  then  be  carefully  ex- 


80  ARMATURE  WINDING  AND  MOTOR  REPAIR 

amined  to  see  that  all  are  straight  and  that  the  openings  at 
the  top  are  wide  enough  to  admit  the  coil  leads  easily.  Test 
the  commutator  for  breakdown  to  ground,  and  for  short- 
circuit  between  segments,  with  the  standard  test  voltage  for 
the  machine.  (See  Chapter  V,  also  page  175.)  All  parts  of 
the  spider  which  come  in  contact  with  the  coils,  such  as  coil 
supports,  etc.,  should  be  carefully  insulated  with  either  tape, 


FIG.  75. — Medium-sized  heavy  duty  armature  showing  use  of  strap  coils 
for  a  wave  winding  (Westinghouse  Electric  &  Mfg.  Company). 

fullerboard  channels,  or  two  or  three  thicknesses  of  fish  paper 
strips.  When  tape  is  used  it  is  wrapped  in  overlapping  layers 
over  the  entire  support.  At  the  point  where  the  spurs  which 
hold  the  coil  support  in  position  prevent  winding  on  the  tape, 
the  iron  should  be  covered  with  insulating  cloth,  and  held  in 
place  by  the  tape  on  each  side.  Each  layer  of  the  tape  should 
be  shellaced  as  it  is  wound.  When  fullerboard  strips  are 
used,  the  first  layer  is  frequently  screwed  to  the  iron  to  pre- 
vent lateral  motion.  Other  layers  are  shellaced  over  this, 


REWINDING  DIRECT-CURRENT  ARMATURES 


81 


and  the  whole  is  usually  bound  with  twine.     Special  care 
should  be  taken  to  stagger  all  joints. 

Inserting  the  Coils. — The  assembly  of  the  different  types  of 
coils  is  essentially  similar.  Mark  two  slots  with  chalk  to 
receive  the  first  coil,  and  count  off  and  mark  the  commutator 
necks  into  which  its  leads  will  be  connected.  Fish  paper  cells 
can  then  be  inserted  into  the  slots,  and  the  coils  driven  into 
position  one  after  another  with  a  mallet  and  a  fiber  drift.  If 
a  two-piece  coil  is  used,  the  lower  half  coils  should  be  inserted 
first  all  the  way  around  the  armature  and  then  the  upper  half 


FIG.  76. — Wave-wound  armature  partly  completed  showing  insulation  used 
between  end  connections  of  coils. 

coils.  If  one  piece  coils  are  used  the  coils  should  be  inserted 
in  regular  succession,  the  bottom  half  of  the  coil  being  driven 
into  the  bottom  of  its  slot  first.  The  other  half  is  driven  into 
close  contact  with  the  coil  which  is  already  in  the  bottom  of 
the  slot.  If  there  is  no  coil  in  the  bottom  of  the  slot,  as  hap- 
pens with  the  throw  coils,  this  top  half  is  inserted  only  tem- 
porarily until  the  winding  has  been  carried  entirely  around  the 
armature.  Then  the  throw  coils  must  be  removed  so  that 
the  coil  sides  can  be  placed  in  the  bottom  of  the  slots. 

When  a  one-piece  coil  is  used  in  a  wave  winding,  the  throw 
coils  span  so  large  a  part  of  the  armature  that  it  is  not  usually 
advisable  to  insert  the  upper  part  of  the  coils  and  then  remove 
them  but  allow  them  to  hang  free  as  shown  in  Fig.  76  until 


82  ARMATURE  WINDING  AND  MOTOR  REPAIR 

all  the  coils  have  one  side  in  place  in  the  slots.  The  upper 
sides  can  then  be  driven  into  place  in  regular  order.  As  in 
the  smaller  windings,  the  upper  and  lower  coil  ends  or  termi- 
nals should  be  separated  with  bands  of  oiled  duck  or  drilling. 
With  one  piece  coils,  this  should  be  threaded  into  place  as  the 
coils  are  inserted  in  the  upper  part  of  the  slots.  With  two- 
piece  coils  it  is  simply  wound  over  the  lower  set  of  coils  before 
the  others  are  placed  in  the  slots. 


FIG.  77. — Core  of  a  large  armature  showing  construction  for  good  ventilation 
(General  Electric  Company}. 

The  coils  must  be  a  close  fit  in  the  slots,  in  order  to  prevent 
any  possibility  of  chafing.  If  necessary,  strips  of  fullerboard 
or  treated  wood  should  be  inserted  at  the  sides  or  bottom  of  the 
slot,  to  make  the  coils  a  tight  fit:  As  each  top  coil  is  put  in 
place  it  should  be  driven  into  the  slot,  the  protecting  cells  then 
cut  off,  and  folded  over  it,  and  fiber  wedges  driven  into  the 
wedge  grooves.  The  slots  on  a  large-sized  machine  are  too 
long  to  allow  one  wedge  to  be  used,  so  that  one  or  more  must 
be  driven  in  from  each  side  of  the  slot  to  furnish  complete 
protection  for  the  face  of  the  coil.  The  armature  should  then 
be  tested  for  grounds,  before  the  connections  are  soldered. 

After  the  winding  is  completed,  the  armature  may  be  banded 
temporarily  at  both  ends.  Then  drive  wooden  wedges  loosely 


REWINDING  DIRECT-CURRENT  ARMATURES 


83 


in  between  the  commutator  necks,  all  around  the  armature  to 
insure  even  spacing.  After  this  drive  them  in  tightly,  to 
force  the  necks  and  coil  ends  into  tight  contact  and  hold  them 
rigidly  in  place.  With  two-piece  coils,  connecting  clips  are 
placed  over  the  leads  at  the  rear  end,  and  wedges  driven  in 
between  these  in  the  same  manner.  The  connections  to  the 
necks,  and  the  rear  end  connections,  if  any  are  used,  can  be 
soldered.  This  soldering  should  be  done  on  the  side  of  the 


FIG.  78. — Large    direct-current    engine  type  armature  showing  method  of 
ventilation  (Westinghouse  Electric  &  Mfg.  Company). 

'machine  instead  of  the  top,  as  a  better  joint  can  be  made  in 
this  manner,  and  there  is  less  liability  of  the  melted  solder 
running  along  the  necks  and  short-circuiting  the  commutator 
segments. 

Before  removing  the  wedges  the  armature  should  be  mounted 
in  a  lathe,  or  if  no  suitable  lathe  is  available,  in  its  bearings 
with  the  field  frame  removed,  and  the  soldered  connections 
turned  down.  If  the  armature  is  mounted  in  its  bearings,  a 
suitable  tool  holder  'must  be  fastened  to  the  frame,  or  some 
rigid  support.  The  commutator  may  then  be  turned  down  and 
given  its  final  polishing  at  the  same  time.  Now  knock  out  the 


84 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


wedges  from  between  the  leads,  and  round  off  the  sharp  cor- 
ners with  a  file.  When  the  bare  copper  is  not  covered  with 
insulation,  insulating  material  can  be  inserted  between  the 
adjacent  leads  to  prevent  accidental  contact.  At  the  rear 
end,  this  usually  takes  the  form  of  asbestos  tape  which  can  be 
interwoven  between  the  leads  or  of  canvas  hoods,  sewed  in 
place.  At  the  front  end,  the  necks  may  be  separated  at 
the  tops  by  strips  of  heavy  fish  paper,  bent  over  the  tops  of  the 
leads  so  as  to  be  held  in  place  by  the  band  wires.  Where  the 


FIQ.  79. — Large   direct-current  engine    type  armature   partly  wound   with* 
strap  coils  (Westinghouse  Electric  &  Mfg.  Company). 

necks  are  quite  long,  additional  separators  may  be  inserted 
half  way  up  from  the  commutator.  These  may  be  in  the  form 
of  fiber  buttons  or  may  merely  consist  of  heavy  twine,  inter- 
woven between  the  long  commutator  necks. 

Banding  Wire. — Bands  of  steel  wire  are  ordinarily  placed 
over  both  ends  of  the  armature,  and  frequently  another  over 
the  connections  to  the  commutator  necks.  No  bands  need  be 
used  over  the  surface  of  the  core,  as  the  wedges  are  sufficient  to 
retain  the  body  of  the  coils  in  place.  The  coils  can  be  pro- 


REWINDING  DIRECT-CURRENT  ARMATURES 


85 


tected  from  the  mechanical  pressure  of  the  banding  by  layers  of 
surgical  tape  separated  by  strips  of  cement  paper,  over  which 
the  bands  are  wound.  The  wire  should  be  wound  on  under 
heavy  tension,  secured  by  clamping  it  between  blocks  of  wood. 
These  blocks  can  be  held  from  moving  by  heavy  straps  of  wire, 
fastened  to  some  rigid  object — usually  the  machine  frame.  If 
desired,  a  spring  balance  may  be  inserted,  which  will  give  the 


FIG.  80. — Armature  of  a  350  kw.,  250-volt,  engine  type  generator  showing 
the  core  construction  for  good  ventilation  and  the  use  of  copper  strap  coils 
(Fairbanks-Morse  &  Company). 

exact  tension  that  is  being  applied.  This  should  run  between 
300  and  400  Ib.  The  bands  should  be  firmly  soldered  in  place. 
When  it  is  desired  to  secure  extra  mechanical  strength, 
the  wire  is  sometimes  wound  on  two  or  three  layers 
deep,  each  layer  being  soldered  separately.  The  proper 
banding  of  a  large  armature  by  this  method  is  often  a  serious 
problem  on  a  repair  job.  A  sectional  band  wire  such  as  shown 
in  Fig.  82,  is  therefore  convenient.  When  using  this  type  of 
banding,  the  two  ends  of  any  section  are  keyed  together  into  an 


86 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


FIG.  81. — Completed  armature  of  a  350  kw.,  250-volt,  direct-current  engine 
type  generator,  lapwound  for  8  poles  with  equipotential  connections.  Four 
coils  per  slot  are  used  (Fairbanks-Morse  &  Company). 


FIG.  82. — Tool  for  use  in  applying  section  bands  on  large  armatures. 


REWINDING  DIRECT-CURRENT  ARMATURES  87 

open  loop  and  then  applied  to  the  armature.  In  making  the 
final  connection  the  special  clamp  shown  in  Fig.  82  is  needed. 
The  two  jaws  of  this  clamp  grip  the  ends  of  the  band  and  by 
means  of  the  handle  whose  lower  end  is  formed  into  a  cam,  the 
jaws  can  be  forced  together  so  as  to  interweave  the  loops  of  the 
band  wire  and  permit  the  steel  key  B  to  be  inserted.  In  this 
operation  the  clamp  can  be  held  in  any  position  by  inserting 


FIG.  83. — Large  direct-current  armature  partly  wound  with  strap  coils. 
Note  the  insulating  strips  between  end  connections  and  double  insulation  in 
slots  (Crocker-Wheeler  Company). 

the  pin  A  through  the  movable  jaw  and  beam.     For  other 
details  of  banding  see  Chapter  VI,  page  146. 

Balancing  Large  Armatures. — After  the  armature  is  banded 
it  is  ready  for  balancing.  Suitable  balancing  ways  may  con- 
sist of  heavy  steel  beams  with  polished  steel  plates  mounted 
on  their  upper  edges.  The  surface  of  the  polished  plates 
must  be  accurately  leveled.  The  shaft  should  rest  on  the 
inner  surface  of  a  polished  steel  ring,  which  in  turn  rests  on 
the  polished  plates.  In  this  way,  an  almost  frictionless 


88 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


bearing  surface  is  obtained,  and  the  armature  tends  to  roll 
until  the  heaviest  part  is  at  the  bottom.  Melted  lead  can  be 
poured  into  recesses  in  the  spider,  or  cast-iron  weights  bolted 
to  the  spider  arms  to  correct  any  unbalanced  condition,  until 
the  armature  will  lie  with  any  part  uppermost.  The  armature 
should  then  be  thoroughly  cleaned  with  an  air  blast  and 


FIG.  84. — Method  of  cross  connecting  a  large  direct-current  armature  having 
a  lap  winding  ( Westinghouse  Electric  &  Mfg.  Company). 

sprayed  inside  and  out  with  black  finishing  varnish.  Special 
care  should  be  taken  to  reach  all  exposed  parts  of  the  core, 
to  prevent  rusting. 

Rotary  Converters. — There  is  no  essential  difference  in  the 
winding  operations  as  described  between  rotary  converters, 
and  other  direct-current  machines.  At  the  rear  of  the  arma- 
ture, however,  taps  are  brought  out  from  the  coils  at  regular 


REWINDING  DIRECT-CURRENT  ARMATURES 


89 


intervals.  On  a  three-phase  machine,  this  will  be  two-thirds 
of  the  pole  pitch;  on  a  two-phase  machine,  one-half  the  pole 
pitch,  and  on  a  six-phase  machine  one-third  the  pole  pitch. 
These  taps  are  connected  to  the  collector  rings.  A  two-phase 
or  a  six-phase  rotary  converter  cannot  be  wound  with  a  wave 
winding  on  account  of  the  necessity  of  having  an  equal  num- 
ber of  coils  between  taps  on  the  armature. 

Three -wire  Generators. — Practically  any  standard  genera- 
tor can  be  adapted  for  use  as  a  three-wire  machine  by  the 


FIG.  85. — Completed  armature  for  a  large  3-wire  direct-current  generator 
(Crocker- Wheeler  Company). 


addition  of  suitable  collector  rings  and  balancing  coils.  These 
coils  are  entirely  self-contained  and  may  be  installed  apart 
from  the  generator.  The  collector  slip  rings  are  usually  much 
smaller  than  those  of  a  rotary  converter,  as  each  one  carries 
only  a  fraction  of  the  unbalanced  current.  The  current 
which  they  carry  is  largely  unidirectional,  only  enough  alter- 
nating current  flowing  to  excite  the  core  of  the  balancing  coils. 
They  are  accordingly  made  of  iron  to  avoid  the  blackening 
from  electrolysis  which  takes  place  when  the  direct  current 
flows  from  copper  to  carbon.  They  may  be  placed  at  either 
end  of  the  armature,  but  are  usually  placed  at  the  end  of  the 
commutator,  for  greater  convenience  (see  Fig.  85) . 


90 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


IV.  WINDING  RAILWAY,  MILL  AND  CRANE  TYPES  OF 
ARMATURES 

The  methods  of  winding  and  insulating  armatures  for  rail- 
way, mill,  mining  and  crane  types  of  armatures  are  practi- 
cally the  same,  since  the  service  is  somewhat  similar  and  the 
windings  must  stand  up  against  much  hard  usage  and  abuse 
of  the  motor.  Thorough  insulation  and  protection  against 
vibration  and  chafing  of  the  insulation  are  essential  points 
in  the  winding  of  these  armatures  and  must  be  given  more 
attention  than  in  any  other. 

Railway  Type  Armature  Coils. — This  type  of  armature  coil 
is  completely  formed  and  impregnated  with  insulating  com- 
pounds. It  needs  no  shaping  when  placed  on  the  armature. 


Fullerboard  Protecting  Strip 
.-'Coil  in  Top  of  Slot  Only 

Plaetlo  Varnish 


Cotton  Tape 


Fuller  Board  Separators 


ullerboard  Protecting  Strip" 
'Coilln  Top  of  Slot  Only 
,D.C.C.  Wire 

„  Treated  Cloth 


Fuller  Board  Separators 


Black  Plastic  Varnish 


Fro.  86. — Wire  wound  coils  show- 
ing single  coils  arranged  in  vertical 
layers. 


|  -^  Cotton  Tape 


FIG.  87. — Wire  wound  coils 
showing  single  coils  arranged  in 
horizontal  layers. 


It  is  usually  advisable  for  the  repairman  to  purchase  these 
coils  from  the  manufacturer  of  the  motor  which  is  to  be  re- 
paired. Usually  the  coils  are  made  up  of  two  or  more  single 
coils  wound  with  round  wire  or  copper  strap.  These  single  coils 
are  separately  wound  but  grouped  together  for  mechanical 
reasons  in  handling  and  insulation.  The  windings  of  these 
motors  are  of  the  wave  or  two-circuit  type,  except  for  the 
largest  sizes  of  mill  or  locomotive  motors.  The  insulation 
for  coils  and  core  and  the  details  of  the  winding  of  railway 
armatures  as  given  here,  are  the  recommendations  outlined  by  a 
writer  in  the  Electric  Journal,  Vol.  VII,  No.  10. 

Wire  Coils. — The  coils  for  smaller  machines  are  made  of 
double  cotton-covered  wire.     The  single  coils  which  form  a 


REWINDING  DIRECT-CURRENT  ARMATURES 


91 


complete  coil  are  insulated  from  each  other  by  fish  paper  or 
fullerboard  separators  and  may  be  arranged  radially  or  cir- 
cumferentially  in  the  slot  as  shown  in  Figs.  86  and  87.  The 
leads  from  wire  wound  coils  are  secured  along  the  diamond  end 
and  leave  the  coil  one  after  the  other,  each  being  firmly  tied 
and  taped  in  position  so  that  there  is  no  possibility  for  them 
to  chafe  against  one  another.  They  are  reinforced  with 
cotton  sleeves. 

Strap  Coils. — Most  of  the  larger  railway  motors  have  the 
armature  coils  made  from  rectangular  conductors  instead 
of  round  wire,  as  with  this  form  of  conductor  a  greater  pro- 
portion of  the  slot  space  may  be  filled  with  copper,  without 


Fish  Paper  Protecting  Strip 
'"Coil  in  Top  of  Slot  Only 


'ullei  board  Protecting  Strip 
Coil  in  Top  of  Slot  Only 


FIG.  88. — Section  of  slot  showing 
strap  wound  coil  of  two  turns.  - 


FIG.  89. — Section  of  slot  showing 
strap  wound  coil  of  one  turn. 


sacrificing  the  insulation  requirements.  The  pressure  on 
the  insulating  surfaces  is  also  more  evenly  distributed,  as 
with  the  round  wire  the  pressure  bears  on  a  line,  while  with 
the  rectangular  conductors  it  is  distributed  over  a  flat  surface 
and  is  much  less  liable  to  injure  the  insulation.  Figs.  88 
and  89  show  a  cross-section  through  two-turn  and  one-turn 
coils,  respectively. 

The  one  turn  coil  readily  lends  itself  to  bringing  out  the 
leads  in  position  to  enter  the  top  and  bottom  of  the  com- 
mutator necks  respectively,  by  the  use  of  the  standard  form 
of  diamond  end.  A  two  turn  coil  requires  a  special  turn  at 
the  rear  end  of  the  coil,  in  order  to  bring  the  leads  out  in 
the  proper  position.  By  the  use  of  this  form  of  coil,  all  the 


92  ARMATURE  WINDING  AND  MOTOR  REPAIR 

advantages  of  the  strip  windi  ig  can  be  secured  for  the  smaller 
machines  on  which  more  than  one  turn  per  slot  is  usually 
required. 

In  some  large  motors,  coils  of  the  rectangular  types  are 
made  in  two  pieces,  and  are  known  as  two-piece  coils.  Their 
advantage  lies  in  the  fact  that  if  a  coil  becomes  damaged,  only 
one-half  of  the  complete  coil  need  be  removed  to  overcome 
the  defect.  As  damage  to  the  coils  nearly  always  occurs  on 
the  outside  of  the  armature,  this  type  of  coil  is  peculiarly 
adapted  to  railway  type  armatures.  It  requires,  however, 
a  soldered  connection  at  the  back,  in  addition  to  the  usual 
soldered  connection  at  the  commutator,  and  hence  is  used 
only  on  the  large  motors  where  the  saving  of  copper  for  repair 
parts  would  be  great. 

Coil  Insulation. — The  insulation  used  on  motors  of  the  type 
under  consideration  depends  largely  on  the  type  of  coil  used. 
Where  double-cotton-covered  coils  are  used  there  is  little 
advantage  in  using  materials  for  the  remainder  of  the  slot 
insulation  which  have  higher  heat  resisting  ability  than  the 
cotton  strands  which  are  in  immediate  contact  with  the  con- 
ductors, since  it  is  necessary  under  the  circumstances  to  limit 
the  temperature  to  values  consistent  with  the  cotton  insulation. 
For  this  reason  on  certain  types  of  mill  motors,  where  the 
temperature  conditions  are  exceptionally  severe,  asbestos 
covering  is  used  instead  of  the  cotton,  with  mica  insulation 
around  the  complete  coil,  cotton  being  used  only  in  the  protec- 
tive taping  over  the  outside. 

With  strap  wound  coils  it  is  possible  to  use  built  up  mica 
in  immediate  contact  with  the  conductors  and  cotton  only 
on  the  outside  protective  coverings,  where  it  is  in  contact  with 
the  air  or  the  relatively  cooler  iron.  Hence  the  copper  can 
be  safely  worked  to  a  higher  value  and  continued  overloads 
and  abuse  will  not  be  so  liable  to  cause  breakdown.  The  coils 
should  be  vacuum  impregnated  before  insertion  in  the  arma- 
ture. This  process  renders  them  thoroughly  moisture  and 
oil  proof  and  prolongs  the  life  of  the  coils  over  that  of  un- 
impregnated  ones,  especially  where  they  are  subject  to  mois- 
ture, acid  fumes  and  deleterious  gases.  It  is  applied  to  all 
railway  type  coils  except  those  for  mine  locomotives,  whose 


REWINDING  DIRECT-CURRENT  ARMATURES 


93 


armatures  are  usually  impregnated  as  a  whole  after  the  wind- 
ing is  completed. 

Insulating  the  Core  of  Railway  Armatures. — In  this  type  of 
armature  it  is  advisable  to  supply  extra  protection  at  every 
point  of  special  electrical  or  mechanical  stress  such  as  where 
the  coils  leave  the  slots,  where  the  leads  leave  the  coils  and 
where  the  leads  cross  one  another  or  cross  the  ends  of  the  coils. 
Before  applying  the  core  insulation,  the  core  should  be 
thoroughly  cleaned  with  an  air  blast  and  burrs  taken  off 
with  a  file.  When  wire  wound  coils  are  used  the  coil  supports 
have  curved  surfaces  and  should  be  insulated  with  treated 
cloth  in  strips  or  with  layers  of  rope  paper  and  treated  cloth. 


Comtnuta'tor 


Coil  Support 

FIG.  90. — Insulation  of  support  for  front  coils  of  railway  armature. 


Slits  should  be  made  in  the  strips  where  necessary  to  make 
them  lie  smooth  as  over  the  end  bell,  care  being  taken  that 
the  slits  in  the  successive  layers  are  staggered.  These  strips 
should  be  bound  together  with  shellac  and  ironed  smoothly 
into  place.  They  should  be  built  up  to  a  thickness  of  about  one- 
eighth  inch  (six  layers  of  rope  paper  and  five  layers  of  treated 
cloth  are  sometimes  used)  over  the  entire  support  and  to  the 
level  of  the  bottom  of  the  slots  and  the  commutator  necks 
at  each  edge.  Where  this  would  require  an  excessive  amount 
of  insulating  material,  as  occurs  in  the  rear  of  the  commutator 
on  certain  types,  a  bed  of  rope  can  be  built  up,  as  shown  in 
Fig.  90,  and  bound  in  place  with  an  insulating  cement.  A  final 
layer  of  friction  tape  can  be  applied  over  all  the  insulation, 
great  care  being  observed  to  make  the  layers  lie  smooth,  and 
to  build  up  a  firm  support  for  the  coils  where  they  leave  the 
slots. 


94  ARMATURE  WINDING  AND  MOTOR  REPAIR 

On  the  cores  for  strap  wound  coils,  the  coil  supports  are 
usually  straight  and  insulated  with  built-up  mica  bushings  or 
with  heavy  bands  of  treated  cement  paper.  No  tape  is  used 
in  this  case,  but  the  bushings  are  arranged  to  come  up  level 
with  the  bottom  of  the  slots. 

On  both  wire  and  strap  wound  armatures,  the  slots  for 
about  an  inch  at  the  ends  may  be  slightly  wider  than  the  coils. 
In  such  cases  narrow  strips  of  heavy  fish  paper,  projecting 
slightly  from  the  slots,  should  be  inserted  for  additional  pro- 
tection to  the  coils  at  this  point.  The  slots  should  be  further 
insulated  with  regular  fish-paper  cells  for  the  mechanical 
protection  of  the  coils. 

Inserting  the  Coils. — Before  starting  to  wind  the  armature, 
the  commutator  should  be  tested  for  breakdown  with  5000 
volts  to  ground  and  200  volts  between  segments.  Mark  two 
slots,  separated  by  the  proper  throw,  with  chalk  for  the  first 
coil.  Then  count  off  from  the  bar  opposite  the  center  of  the 
first  slot,  the  commutator  bars  into  which  the  leads  from  these 
slots  must  fit.  In  a  lap  winding  these  bars  should  lie  adjacent. 
In  a  wave  winding,  the  number  of  bars  between  them  must  be 
determined.  (See  page  105,  Chapter  IV.)  The  first  coil  is 
then  placed  in  these  two  slots,  the  bottom  half  being  driven 
into  the  lower  half  of  the  slot,  and  the  top  half  being  merely 
caught  in  its  proper  slot,  as  it  will  have  to  be  removed  later, 
to  allow  a  coil  to  be  inserted  beneath  it. 

Wire  Coils. — In  wire  wound  armatures  the  lower  leads 
should  be  taped  with  friction  tape  when  necessary  to  make 
the  insulation  continuous  from  the  coil  to  the  commutator. 
These  leads  should  then  be  laid  along  the  coil  supports  of  the 
armature  core  in  smoothly  fitting  rows  and  the  bare  ends 
driven  into  the  proper  commutator  necks.  Heavy  insulation 
is  needed  between  the  coil  ends  and  the  upper  and  lower 
leads  and  between  the  upper  and  lower  coil  ends.  This  may 
be  of  different  form  in  different  types  of  armatures.  In  one 
type  treated  canvas  strips  may  be  inserted  so  as  to  furnish 
extra  insulation  between  the  ends  of  adjacent  coils  and  be- 
tween the  coil  ends  and  the  lower  leads  In  addition  a  friction 
cloth  strip,  doubled  oVer  a  piece  of  rope,  may  be  inserted  at 
each  end  between  the  upper  and  lower  coil  as  the  coils  are 


REWINDING  DIRECT-CURRENT  ARMATURES 


95 


inserted,  the  rope  fitting  in  the  point  of  the  diamond.  The 
coils  should  be  shaped  with  a  fibre  drift  and  rawhide  mallet 
so  as  to  fit  snugly  against  one  another  at  both  ends.  It  is 


FIG.  91. — Inserting  the  top  sides  of  coils  in  an  armature.  The  leads  of 
the  bottom  coil  sides  are  shown  connected  to  the  commutator.  Note  the 
band  of  insulation  between  end  connections. 


essential  that  they  be  made  to  fit  closely  together  when  first 
inserted,  otherwise  the  armature  will  bulge  at  the  ends.  Any 
attempt  to  shape  the  coils  in  a  completed  armature  is  liable 
to  injure  the  insulation. 


96  ARMATURE  WINDING  AND  MOTOR  REPAIR 

After  all  the  coils  have  been  inserted  and  the  top  parts  of 
the  throw  coils  have  been  replaced,  the  ends  of  the  canvas 
strips  which  project  up  between  the  coil  ends  should  be  trim- 
med off  level  with  the  top  of  the  coils  at  both  ends  of  the  arma- 
ture. Those  strips  which  project  out  from  beneath  the 
coils,  should  be  turned  up  over  the  coil  ends  and  bound  in 
place  with  friction  tape.  This  tape  when  wound  completely 
across  the  upper  surface  of  the  coil  ends,  serves  as  a  protecting 
and  insulating  layer  between  the  coils  and  the  upper  leads. 

Another  method  of  providing  extra  insulation  on  some 
armatures  is  to  slit  two  strips  of  treated  canvas  and  insert 
them  between  the  lower  leads  and  the  coil  ends  with  the  slits 
staggered.  No  strips  are  inserted  at  the  rear  end  and  no 
insulation  is  required  between  the  ends  of  adjacent  coils, 
beyond  that  on  the  coils  themselves.  Strips  of  friction  cloth 
and  rope  may  be  inserted  between  the  upper  and  the  lower 
coil  ends,  and  the  canvas  strips  folded  over  the  ends  of  the 
coils  and  covered  with  friction  tape  as  just  described.  In 
this  case,  however,  strips  of  fish  paper  should  be  slipped  over 
each  coil  and  wedged  between  the  coil  ends  close  to  the  core, 
for  further  protection  to  the  upper  leads. 

Before  connecting  the  upper  leads  to  the  armature  they 
should  all  be  tied  together  with  bare  copper  wire  and  the  coils 
subjected  to  a  break-down  test  of  3600  volts.  Any  defective 
coil  must  be  replaced.  The  armature  can  then  be  trued  up. 
This  can  be  done  by  tapping  down  with  a  mallet  all  the  high 
coils  as  located  by  holding  a  piece  of  chalk  so  that  it  will  rub 
against  the  high  parts  when  the  armature  is  revolved.  A  some- 
what better  method  is  to  squeeze  the  coils  into  place  by  means 
of  a  flexible  metallic  strap  and  turn-buckle.  The  latter 
method  is  less  liable  to  damage  the  insulation  and  all  the  coils 
receive  uniform  treatment  so  that  a  better  balance  of  the 
armature  results.  Both  ends  must  form  a  compact  mass. 
Where  end  room  is  especially  short,  as  in  mine  motor?,  a  special 
form  can  be  used  on  the  rear  end  of  the  armature  so  as  to 
press  all  coils  against  this  when  being  inserted  to  give  uni- 
formity to  the  arrangement. 

Where  the  top  of  the  coil  is  above  the  top  of  the  commutator, 
there  is  sometimes  difficulty  in  keeping  the  leads  properly 


REWINDING  DIRECT-CURRENT  ARMATURES  97 

separated  in  bringing  them  down  to  the  commutator.  In 
such  cases  a  canvas  strip  may  be  interwoven  over  every  other 
one,  making  it  possible  to  have  two  layers  of  leads  on  the 
vertical  part.  The  leads  are  then  inserted  into  the  slits  in  the 
proper  commutator  necks  and  copper  wire  dummies  driven 
over  them,  to  prevent  any  possibility  of  a  portion  of  the  leads 
being  removed  when  the  necks  are  turned  down.  Both  leads 
and  dummies  should  be  tinned  and  make  a  driving  fit  in  the 
necks. 

Strap  Coils. — The  leads  of  strap  wound  armatures  are 
formed  to  shape  and,  therefore,  require  little  bending  during 
their  installation.  The  coil  supports  of  the  two  turn  strap 
coils  are  shaped  and  insulated  in  a  manner  very  similar  to 
that  for  wire  wound  coils.  In  addition  a  bed  of  the  insulating 
cement  is  made  over  the  insulation  at  the  rear  of  the  commuta- 
tor into  which  the  lower  leads  are  forced  as  they  are  inserted 
into  the  commutator  necks.  They  are  thus  held  rigidly  in 
place  after  the  paste  hardens.  Strips  of  treated  canvas  or 
of  friction  cloth  folded  over  fish  papsr  and  mica  should  be 
threaded  between  the  upper  and  lower  coil  ends,  at  each  side 
of  the  machine,  as  the  coils  are  inserted.  A  length  of  rope 
should  also  be  threaded  through  the  diamond  point  at  each 
end.  Between  the  coil  ends  and  the  upper  and  lower  leads, 
strips  of  treated  canvas  with  slit  edges  should  be  inserted 
so  that  the  openings  will  be  staggered.  The  edge  toward 
the  core  must  be  shaped  to  fit  up  between  the  coils  and  furnish 
added  protection  to  the  leads.  After  all  the  coils  have  been 
inserted,  the  upper  leads*  may  be  bent  up  slightly,  and  the 
edges  of  the  various  insulation  strips  cut  off  even  with  the 
commutator.  Friction  tape  should  then  be  wound  smoothly 
over  the  treated  canvas,  holding  it  in  place,  and  forming  a  bed 
for  the  upper  leads.  These  leads  can  then  be  bent  down  and 
inserted  into  the  proper  commutator  necks. 

Coil  supports  for  one-turn  strap  coils,  whether  two  piece  or 
one  piece,  are  straight,  and  can  be  insulated  with  built  up  mica 
forms,  or  strips  of  fish  paper  or  treated  cement  paper,  shellaced 
and  tied  in  place.  Insulating  cement  should  be  plastered 
over  the  insulation  back  of  the  commutator,  to  hold  the 
leads  from  the  individual  coils  in  place.  Separate  the  ends 

7 


98  ARMATURE  WINDING  AND  MOTOR  REPAIR 

of  the  coils  by  two  thicknesses  of  treated  canvas,  threaded  in 
place  as  the  coils  are  inserted. 

The  coil  supports  for  two-piece  coils  may  be  insulated  in  the 
same  manner  as  the  one-piece  coils.  The  end  bells  for  this 
type  of  armature,  however,  are  usually  separate  from  the  core 
and  are  not  put  in  place  until  the  winding  is  complete,  so  that 
the  winder  has  plenty  of  room  to  work  on  the  rear  end  of  the 
coils.  The  winding  operation  can  be  greatly  facilitated  by 
the  use  of  a  steel  winding  jig,  consisting  of  a  slotted  disk  with 
a  hub  bored  to  fit  the  armature  shaft.  The  number  of  slots 
should  be  equal  to  the  number  of  single  coils  in  the  armature, 
and  the  thickness  of  the  disk  equal  to  the  width  of  the  connect- 
ing clips.  As  each  coil  is  placed  in  the  armature  slot,  the 
straps  composing  it  should  be  placed  in  the  proper  slots  in  the 
jig  and  in  the  commutator  necks  until  all  the  lower  half  of  the 
winding  is  in  place.  The  leads  on  the  upper  and  the  lower 
half  should  be  separated  by  a  couple  of  thicknesses  of  treated 
cement  paper,  or  by  a  layer  of  fish  paper  and  mica.  After  all 
the  coils  are  in  place,  the  straps  which  are  to  be  connected 
together  at  the  rear  end  lie  one  above  the  other  in  the  slots 
of  the  jig.  These  can  be  cut  off  even  with  the  surface  of  the 
jig,  a  temporary  band  wrapped  around  the  coil  ends,  and  the 
jig  removed.  Copper  connector  sleeves  should  then  be  slipped 
over  the  coil  ends  and  wooden  wedges  driven  in  between  them. 
The  connectors  may  then  be  soldered  and  the  coil  ends  turned 
down  at  the  top  and  side.  Next  knock  out  the  wedges  and 
interweave  asbestos  braid  between  the  connectors  to  prevent 
accidental  contact.  The  end  bell,  •  properly  insulated  with 
moulded  mica  or  moulded  paper,  can  then  be  bolted  into 
position. 

Connections  with  Dead  Coils. — If  in  a  four-pole  motor  with 
a  two-circuit  winding  there  is  an  even  number  of  single  coils 
in  a  complete  coil,  one  single  coil  in  the  armature  must  be  cut 
out  in  order  that  the  winding  may  be  made  continuous.  This 
coil  is  called  a  dead  coil,  because  it  is  not  connected  to  the 
circuit.  It  is  necessary  first  to  determine  the  number  of  single 
coils  in  a  complete  coil,  by  dividing  the  number  of  complete 
coils  or  slots  by  the  number  of  commutator  bars.  If  there  are 
more  leads  from  each  side  of  the  coil  than  there  are  single 


REWINDING  DIRECT-CURRENT  ARMATURES  99 

coils,  each  single  coil  is  composed  of  two  or  more  wires  in 
parallel  and  these  must  be  treated  as  a  single  lead.  In  strap 
coils  each  strap  corresponds -to  a  single  coil.  The  coil  is  cut 
out  by  cutting  off  the  leads  on  both  sides  of  a  single  coil  about 
an  inch  from  where  they  separate  from  the  coil.  Then  care- 
fully tape  them  up.  The  body  of  the  coil  of  course,  must  be 
left  in  the  slot  for  uniformity  of  the  winding. 

Hooding  and  Banding. — As  a  final  protection  to  the  arma- 
ture coils,  heavy  hoods  may  be  put  on  over  the  ends  of  the 
coils,  covering  the  armature  from  the  commutator  to  the  core 
and  from  the  core  to  the  end  bell.  At  the  commutator  end 
the  hood  may  be  of  woven  asbestos  sewed  to  a  conical  shape, 
and  impregnated  in  a  moisture  and  oil  repelling  compound. 
It  should  be  put  in  place  while  wet.  The  small  end  should  be 
drawn  up  over  the  commutator,  turned  inside  out,  and  firmly 
tied  over  the  leads  and  commutator  necks  with  heavy  twine. 
The  body  of  the  hood  must  then  be  turned  back  over  the  arma- 
ture. If  the  commutator  necks  are  lower  than  the  level  of  the 
core,  another  layer  of  twine  should  be  wound  over  the  hood 
near  the  commutator  and  a  band  of  canvas  sewed  over  the 
whole.  Then  stretch  the  hood  tightly  back  over  the  armature 
and  tie  with  twine. 

Around  the  rear  end  of  the  armature,  a  band  of  canvas  may 
be  wrapped  so  that  the  greater  part  of  the  strip  extends  out 
over  the  shaft,  only  enough  being  wound  over  the  armature  to 
permit  a  secure  fastening.  This  may  be  bound  in  place  with 
a  band  of  twine  wound  tightly  in  the  groove  between  the  coil 
ends  and  the  end  bell.  The  canvas  should  then  be  turned 
back  over  the  armature  and  bound  smoothly  in  place. 

The  number  and  size  of  bands  depends  upon  the  size  and 
speed  of  the  armature.  All  armatures  should  have  bands  on 
each  end,  placed  as  far  out  as  possible,  so  as  to  cover  the 
greater  part  of  the  coil  ends.  When  such  a  band  would  be 
quite  wide,  two  separate  narrower  bands  may  be  wound  on 
each  end.  These  bands  should  be  insulated  from  the  coils 
by  three  turns  of  canvas  tape  separated  by  treated  paper 
which  extends  at  least  one-eighth  inch  beyond  the  band  on 
each  side.  The  bands  around  the  body  of  the  core  in  the  band 
grooves  should  be  insulated  from  the  core  and  coils  by  strips 


100         ARMATURE  WINDING  AND  MOTOR  REPAIR 

of  fish  paper.  They  should  also  be  put  on  with  a  tension 
of  about  350  pounds  sufficient  to  make  a  good  firm  band  and  to 
bring  the  coils  down  so  that  they  will  not  project  above  the 
surface  of  the  core  at  any  point.  The  individual  turns  of  the 
banding  should  be  well  soldered  together  and  held  at  several 
places  by  thin  copper  clips-  placed  before  the  banding  was 
started.  For  other  details  of  banding  see  Chapter  V,  page 
146. 

After  the  banding  is  completed,  the  mica  insulation  between 
the  commutator  bars  should  be  undercut  to  a  depth  of  about 
one-sixteenth  inch,  with  a  special  milling  cutter.  The  entire 
armature  except  the  commutator,  can  now  be  sprayed  with  an 
air-drying  finishing  varnish. 


CHAPTER  IV 
MAKING  CONNECTIONS  TO  THE  COMMUTATOR 

Before  starting  to  insert  coils  in  the  slots  of  the  armature, 
the  commutator  should  be  tested  for  grounds.  This  can  be 
done  by  touching  one  lead  of  a  high-voltage  transformer 
(1200  to  2000  volts)  to  the  shaft  and  moving  the  other  over  the 
surface  of  the  commutator  and  at  the  edges.  If  there  is  no 
arcing,  the  commutator  is  properly  insulated.  The  winder 
can  now  insert  the  coils.  As  each  coil  is  put  in  place  the 
sleeving  on  the  ends  of  the  lower  leads  should  be  fastened  to  the 
wire  by  a  few  turns  of  friction  tape  and  these  leads  inserted 
into  the  slits  of  the  proper  commutator  bars.  In  case  the 
coil  has  two  or  more  start  and  finish  ends,  care  must  be  taken 
that  the  different-colored  sleeves  are  connected  to  the  com- 
mutator always  in  the  same  order. 

Locating  First  Connection  to  Commutator. — Before  con- 
necting the  first  coil  to  the  commutator,  the  winder  must 
examine  the  setting  of  the  brushes,  to  see  whether  they  are 
centered  between  pole  tips  or  opposite  the  center  of  the  pole. 
When  the  brushes  are  centered  between  the  pole  tips,  the 
start  end  of  each  coil  must  be  connected  straight  out  to  the 
bar  opposite  the  slot  in  which  the  beginning  of  the  coil  is 
located.  When  the  brushes  are  opposite  the  center  of  the  poles, 
the  start  end  of  the  coil  must  be  swung  a  certain  number  of 
bars  (equal  to  90  electrical  degrees)  to  the  right  or  left  of  the 
bar  opposite  the  slot  in  which  the  beginning  or  bottom  side 
of  the  coil  is  located.  The  number  of  bars  right  or  left  (equal 
to  90  electrical  degrees)  can  be  determined  by  the  following 
formula:  Total  number  of  commutator  bars  -r-  (Number  of 
poles  X  2).  If  this  number  is  mixed  such  as  6.5,  use  the  next 
higher  whole  number  as  7.  The  reason  for  these  connections 
of  leads  is  that  the  coils  must  be  commutated  or  short -cir- 

101 


102          ARMATURE  WINDING  AND  MOTOR  REPAIR 

cuited  by  the  brushes  while  the  coil  sides  are  in  a  neutral 
position  or  outside  the  pole  flux. 

The  spacing  of  the  brushes  from  the  heel  of  one  brush  to  the 
heel  of  the  next  for  a  lap  winding  will  be  equal  to  the  num- 
ber of  commutator  bars  divided  by  the  number  of  poles. 

Testing  Out  Coil  Terminals. — After  the  commutator  pitch 
has  been  determined,  take  the  bottom  lead  of  one  coil  and 
connect  it  to  a  commutator  riser,  using  the  same  throw  as 
employed  on  the  old  winding  in  case  it  is  being  duplicated. 
After  the  position  of  the  first  lead  has  been  determined,  the 
remainder  of  the  bottom  leads  can  be  connected  in  rotation. 
When  all  the  bottom  leads  are  in  place,  a  lighting  out  test 
should  be  made  to  see  if  the  leads  are  connected  to  the  proper 
bars.  A  short  circuit  and  ground  test  must  also  be  made  at 
this  time. 

For  a  lighting-out  test,  place  one  terminal  of  the  lamp  tester 
on  a  commutator  bar,  and  with  the  other  touch  the  top  leads 
of  several  coils  until  the  lamp  lights.  This  will  locate  the  top 
side  of  the  coil  corresponding  to  the  bottom  side  connected 
to  the  test  lamp.  If  the  lamp  lights  on  more  than  one  lead, 
it  indicates  a  short  circuit  between  coils.  In  an  armature 
containing  twice  the  number  of  commutators  bars  as  there 
are  armature  slots,  the  same  procedure  is  followed  in  locating 
the  two  leads  of  the  same  coil. 

Commutator  Connections  for  a  Lap  Winding. — After  all 
the  beginning  ends  of  the  coils  or  the  ends  for  the  coil  sides  in 
the  bottom  of  the  slot  have  been  connected  to  the  commutator 
and  all  the  coils  tested  out  for  open  circuit,  short  circuit 
and  grounds,  the  finish  ends  or  the  ends  of  the  coil  sides  in 
the  top  of  the  slot,  can  be  connected  to  the  commutator  In 
the  case  of  a  coil  wound  with  one  wire  or  two  wires  in  parallel, 
the  finish  end  of  coil  No.  1  is  connected  to  the  commutator 
bar  next  to  that  to  which  the  start  end  is  connected.  Thus, 
if  for  coil  No.  1  the  beginning  end  is  connected  to  bar  No.  1 
the  finish  end  will  be  connected  to  bar  No.  2.  The  beginning 
end  of  coil  No.  2  will  also  be  connected  to  bar  No.  2 
and  its  finish  end  to  bar  No.  3  and  so  on  until  for  the  last 
coil  the  finish  end  will  connect  to  bar  No.  1  and  close  the  wind- 
ing (see  Fig.  97). 


MAKING  CONNECTIONS  TO  THE  COMMUTATOR     103 

On  the  armature  of  some  large  direct-current  machines 
a  commutator  is  provided  which  has  twice  or  three  times  as 
many  commutator  bars  as  slots  or  winding  coils.  The  object 
of  this  is  to  improve  commutation  and  prevent  sparking  by 
commutating  only  a  part  of  the  current  at  a  time.  In  a  case 
where  the  commutator  has  twice  as  many  bars  as  slots  the 
coils  are  made  up  with  two  wires-in-hand  when  winding. 
For  the  case  where  there  are  three  times  as  many  bars  as  slots, 
coils  made  up  with  three  wires-in-hand  when  winding  are  used. 
The  coils  in  either  case  are  placed  in  the  slots  the  same  as 
those  wound  with  one  wire  but  connected  to  the  commutator 
differently. 

In  the  case  where  the  number  of  bars  is  double  the  number  of 
slots  and  coils,  the  start  and  finish  ends  of  the  coils  will  have 
two  wires.  For  coil  No.  1  connect  the  start  ends  as  follows: 
One  wire  to  bar  No.  1  and  the  other  end  to  bar  No.  2;  for 
coil  No.  2,  one  end  to  bar  No.  3,  and  the  other  to  bar  No.  4; 
and  so  on.  When  connecting  the  finish  ends  proceed  as 
follows:  For  coil  No.  1,  connect  one  end  to  bar  No.  3  and  the 
other  to  bar  No.  4;  for  coil  No.  2,  one  end  to  bar  No.  5  and  the 
other  end  to  bar  No.  6  and  so  on  until  for  the  last  coil  one  of 
the  finish  ends  will  connect  to  bar  No.  1  and  the  other  to  bar 
No.  2.  In  this  case,  each  brush  will  cover  two  bars. 

When  there  are  three  start  and  three  finish  ends  to  each 
coil  and  three  times  as  many  commutator  bars  as  slots, 
proceed  in  the  same  way  by  connecting  the  start  ends  to 
adjacent  bars  and  the  end  of  one  coil  to  the  start  of  the  next. 
In  this  case  each  brush  will  cover  three  bars. 

Requirements  of  a  Lap  Winding. — A  lap  winding  can  be 
wound  on  an  armature  having  any  number  of  slots  provided 
each  slot  will  accommodate  two  coil  sides  for  each  commutator 
bar  and  the  number  of  bars  is  an  even  multiple  of  the  number 
of  slots.  When  the  total  number  of  coils  is  not  exactly  di- 
visible by  the  number  of  pairs  of  poles  the  winding  pitch 
can  not  be  equal  to  a  pole  pitch.  This,  however,  is  not  an 
obstacle  except  where  it  is  desirable  to  use  equipotential 
connectors.  When  the  winding  pitch  can  not  be  equal  to  a 
pole  pitch  it  is  made  so  as  nearly  as  possible.  When  the  front 
and  back  pitches  are  specified  to  be  odd  for  a  lap  winding, 


104 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


the  coil  pitch  in  winding  spaces  is  meant  and  not  the  slot 
pitch.  It  will  be  found  that  with  the  usual  type  of  form 
wound  coils,  the  pitches  for  back  and  front  are  necessarily 
odd,  because  one  side  of  the  coil  is  in  the  bottom  of  a  slot 
and  the  other  side  in  the  top  of  the  slot.  The  terminals  of  the 
coil  side  in  the  bottom  of  the  slot  are  usually  considered  the 
start  ends  and  the  terminals  of  the  side  in  the  top  of  the  slot 
the  finish  ends. 

Commutator  Connections  for  a  Wave -Winding. — For  a 
wave  winding,  the  start  and  finish  ends  of  coils  are  connected 
to  the  commutator  differently  than  for  a  lap  winding.  In 
the  case  of  a  coil  wound  with  one  wire,  having  one  start  and 
one  finish  end,  these  are  not  connected  to  adj  acent  commutator 
bars  as  in  a  lap  winding,  but  a  number  of  bars  apart.  When 
the  coils  are  inserted  in  the  slots  the  start  ends  of  the  coil 

sides  in  the  bottom  of  the 
slots  should  be  connected 
to  the  commutator  as  in 
the  lap  winding  when  each 
coil  is  placed.  When  ready 
to  connect  the  ends,  the 
commutator  pitch  must  be 
determined.  The  formula 
for  commutation  pitch  in 
numbers  of  bars  is  (yk  = 
2/i  +  2/2)  -5-  2. 

Where    2/1    is   the   front 

FIG.  92. — A  4-pole,  wave  winding  for     pitch    and    2/2   is    the    back 

pitch  of  the  coil,  each 
counted  in  winding  spaces. 
With  a  double  layer  winding  or  two  coils  per  slot,  there  will 
be  two  winding  spaces  per  slot. 

In  the  case  of  a  4-pole,  double  layer  with  13  slots  and  13 
commutator  bars,  2/2  equals  7  and  2/1  equals  5.  Then  yk  = 
(5  +  7)  -T-  2  or  six  bars.  With  a  back  pitch  of  7  winding 
spaces  and  a  front  pitch  of  5  winding  spaces  coil  No.  1  would 
lie  in  slot  No.  1  and  slot  No.  4.  Its  start  end  would  be  con- 
nected to  commutator  bar  No.  1  and  its  finish  end  to  bar 
No.  7.  Coil  No.  2  would  he  in  slots  No.  2  and  5.  Its  start 


an  armature  having  13  slots  and  13  com- 
mutator bars. 


MAKING  CONNECTIONS  TO  THE  COMMUTATOR     105 


end  would  be  connected  to  bar  No.  2  and  its  finish  end  to  bar 
No.  8  and  so  on  until  coil  No.  13  in  slots  NGS.  13  and  3  would 
have  its  start  end  connected  to  bar  No.  13  and  its  finish  end 
to  bar  No.  6. 

It  will  thus  be  seen  that  in  a  4-pole  machine  the  finish  of 
one  coil  is  joined  to  the  commutator  and  to  the  start  of  another 
which  lies  under  another  pair  of  poles.  (See  Fig.  92.)  The 
finish  end  of  the  latter  coil  is  connected  to  the  commutator 
bar  adjacent  to  the  one  to  which  the  start  of  the  former  coil  is 
connected.  In  Fig.  92  it  can  be  seen  that  the  coils  referred 
to  are  the  ones  connected  to  commutator  bars,  No.  1,  7  and  13. 
In  other  words  the  two  coils  are  connected  in  series  with 
their  start  ends  under  different  north  poles  and  their  finish 
ends  under  different  south  poles.  This  explains  why  the 
wave  winding  is  called  a  series  or  two  circuit  winding. 


Top. 


25  Slot. 
^123  Bars 
"Ooil  Slots  1  and  7 

Lead  Bart  1  and  «W 


FIG.  93. — Wave  winding  with  an  odd  coil  throw  and  an  odd  throw  of  coil 

leads. 

Practical  Method  for  Locating  First  Connection  to  Com- 
mutator for  a  Wave  Winding. — When  there  are  an  odd  num- 
ber of  leads  per  coil,  or  in  case  of  a  dead  coil  an  odd  number  of 
coils  remain,  the  following  procedure  will  locate  the  first 


106 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


connection  to  the  commutator.  Locate  the  center  line  be- 
tween the  slots  of  the  coil  throw.  If  the  throw  of  the  leads  of 
the  coils  is  an  odd  number  of  bars,  this  center  line  will  fall  on 
a  commutator  bar.  If  it  is  an  even  number,  the  center  line 
will  fall  on  the  mica  between  bars.  In  either  case  the  bar  or 
mica  so  located  is  the  starting  point  for  laying  off  the  connec- 
tions to  the  commutator. 

If  there  are  an  odd  number  of  bars  in  the  throw  of  the  leads, 
take  one  less  than  the  number  of  bars  and  count  off  half  of  this 


FIG.  94. — Wave  winding  with  an  even  coil  throw  and  an  even  throw  of  coil 

leads. 

number  in  each  direction  from  the  starting  bar,  and  this  will 
give  the  first  and  last  bar  of  the  commutator  throw.  If 
there  is  an  even  number  of  bars  in  the  throw,  count  off  half 
the  number  in  each  direction  from  the  starting  mica.  A 
check  is  to  count  from  the  first  to  the  last  bar,  and  see  if  it 
agrees  with  the  information  given.  As  the  first  coil  put  down 
will  have  an  odd  number  of  leads,  the  center  one  of  the  top  and 
bottom  leads  should  be  placed  in  the  first  and  last  bar  of  the 
throw  as  determined. 


MAKING  CONNECTIONS  TO  THE  COMMUTATOR     107 

When  there  are  an  odd  number  of  coils  per  slot  and  an  even 
number  of  slots,  a  somewhat  different  method  is  required. 
This,  however,  seldom  occurs.  In  such  a  case,  if  the  lead 
throw  is  an  odd  number  of  bars,  the  center,  as  indicated  by  the 
coil  throw,  will  line  up  on  the  mica  and,  if  an  even  number  of 
bars,  it  will  line  up  on  a  bar.  If  there  is  an  odd  number  of 
bars  in  the  throw,  take  one  less  than  the  number  of  bars  and 
count  off  half  this  number  to  the  left  and  one  more  than  half 
to  the  right,  and  this  will  give  first  and  last  bar  of  the  commuta- 
tor throw.  If  there  is  an  even  number  of  bars  in  the  throw 
count  off  half  the  throw  to  the  right  and  one  less  than  half  to 
the  left.  If  there  are  two  leads  in  the  first  coil,  No.  1  lead 
should  lie  in  No.  1  bar,  and  if  there  are  four  leads  in  the  first 
coil,  No.  2  lead  should  lie  in  No.  1  bar. 


Winding  conditions  with  odd  number 
of  commutator  bars 

Center  line  of  coil  lines  up 

with 

Coil  throw  and  lead  throw  even 

Mica  and  tooth 

Coil  throw  odd,  lead  throw  odd  

Bar  and  slot 

Coil  throw  even,  lead  throw  odd  .... 

Bar  and  tooth 

Coil  throw  odd,  lead  throw  even 

Mica  and  slot 

Requirements  for  a  Wave  Winding. — For  the  wave  or  two- 
circuit  winding  there  should  always  be  an  odd  number  of 
commutator  bars.  With  an  odd  number  of  slots  on  the  arma- 
ture core,  and  an  odd  number  of  coil  sides  per  slot,  a  balanced 
wave  winding  with  no  dead  coils  is  possible.  The  actual 
number  of  coil  sides  per  slot  can  be  determined  by  counting 
the  terminals  of  each  taped  up  coil  in  the  slot.  There  may  be 
only  two  taped-up  coil  sides  in  the  slot  but  each  coil  may  be 
wound  with  three  wires  in-hand  while  winding.  In  such  a 
case  there  will  be  six  coil  sides  per  slot  for  a  two-layer  winding. 

Progressive  and  Retrogressive  Wave  Windings. — When  a 
wave  winding  passes  once  around  the  complete  armature  and 
has  its  start  and  finish  ends  connected  to  the  commutator  as 
shown  in  Fig.  95  (at  left),  it  is  said  to  be  a  progressive  winding. 
When  after  passing  once  around  the  armature  the  start  and 
finish  ends  are  connected  as  shown  in  Fig.  95  (at  right)  it  is 
said  to  be  retrogressive. 


108         ARMATURE  WINDING  AND  MOTOR  REPAI"R 

The  conditions  under  which  a  wave  or  series  winding  can  be 

kp 
used  are  shown  by  the  formula:  K  =  -^±  1. 

Where  K  is  the  number  of  commutator  bars,  p  the  number  of 
poles  and  k  a  whole  number.  When  the  minus  sign  is  used 
the  winding  will  be  progressive  and  when  the  plus  sign  is  used 
it  will  be  retrogressive  in  a  case  where  the  number  of  com- 
mutator bars  is  odd. 


FIG.  95. — The  illustration  at  the  left  shows  one  turn  of  a  progressive  wave 
winding;  the  one  at  the  right  a  retrogressive  wave  winding. 

Wave  Winding  with  Dead  Coils. — For  a  wave  winding,  as 
already  mentioned,  the  number  of  commutator  bars  must 
not  be  a  multiple  of  the  number  of  poles,  otherwise  the  wind- 
ing would  close  after  it  has  passed  once  around  the  armature, 
instead  of  advancing  or  falling  behind  one  commutator  bar 
as  required  by  the  wave  winding.  In  other  words,  there  must 
be  as  many  coils  in  series  between  adjacent  commutator  bars 
as  there  are  pairs  of  poles.  Also  with  two  or  more  coil  sides 
per  slot,  the  total  winding  pitch  of  the  end  connections  (front 
pitch  +  back  pitch)  should  be  a  whole  number.  It  frequently 
happens  that  is  found  to  be  a  fraction.  By  dropping  the 
fraction  when  the  number  is  odd,  a  wave  winding  can  usually 
be  connected  with  a  dead  coil.  A  rule  which  is  often  used  by 
engineers  to  determine  when  a  wave  winding  is  possible  with- 
out a  dead  coil  is  that  given  in  another  paragraph,  namely: 

kp 

Number  commutator  bars  =  -^  ±  1 ;  where  p  is  the  num- 
ber of  poles  and  k  must  be  a  whole  number.  When  an  even 
number  of  coil  sides  per  slot  are  used  and  an  odd  number  of 


MAKING  CONNECTIONS  TO  THE  COMMUTATOR     109 

commutator  bars,  there  will  always  be  a  dead  coil.  Also 
when  there  are  an  even  number  of  slots  on  the  armature 
core,  an  odd  number  of  commutator  bars,  and  an  odd 
number  of  coil  sides  per  slot  are  used,  there  will  always  be  a 
dead  coil. 

Another  rule  for  a  possible  wave  winding  is  that  the  total 
number  of  slots  times  the  number  of  coil  sides  per  slot  may 
be  any  number  divisible  by  the  number  of  poles. 

When  it  is  found  that  a  dead  coil  must  be  used,  one  coil, 
(any  one)  on  the  armature  should  be  cut  back  a  short  distance 
from  the  commutator  and  taped  up  with  friction  tape.  The 
other  coils  can  then  be  connected  as  in  any  other  waye  winding. 
That  is,  connect  all  the  leads  of  the  bottom  sides  of  the  coils 
to  the  commutator  first,  then  connect  the  leads  of  the  top 
sides  of  the  coils  the  proper  commutator  pitch  away  from  the 
bottom  leads. 

When  there  are  twice  as  many  commutator  bars  as  coils  in  a 
wave  winding,  each  coil  is  wound  with  two  wires  and  there  are 
two  start  ends  and  two  finish  ends.  In  making  connections 
to  the  commutator  the  two  start  ends  should  be  connected  to 
adjacent  bars  and  the  two  finish  ends  to  adjacent  bars,  spaced 
the  proper  commutator  pitch  apart. 

Cutting  Out  Coils  of  a  Retrogressive  and  Progressive  Wave 
Winding. — In  Fig.  96a  is  shown  a  portion  of  the  winding  diagram 
of  the  armature,  for  the  case  of  a  retrogressive  winding  with  a 
connecting  pitch  of  1  to  49.  If  the  burned  out  coil  is  between 
commutator  bars  47  and  95,  disconnect  it  from  the  commutator 
at  a  and  b  and  cut  it  at  c  if  it.  has  more  than  one  turn.  Then 
connect  commutator  bars  46  to  47,  and  94  to  95  and  disconnect 
the  coil  between  bars  95  and  46  at  a'  and  b'.  Or  connect  bar 

95  to  96  and  47  to  48  and  disconnect  the  coil  between  bars 

96  and  47  at  a"  and  b" '. 

For  the  case  of  a  progressive  winding  with  a  connecting  pitch 
of  1  to  50,  the  winding  diagram  is  shown  in  Fig.  966.  If  the 
burned  out  coil  is  between  commutator  bars  3  and  52,  dis- 
connect it  from  the  commutator  at  a  and  6  and  cut  it  at  c, 
if  it  has  more  than  one  turn.  Then  connect  bars  3  to  4  and 
52  to  53  and  disconnect  the  coil  between  commutator  bars 
52  and  4  at  a'  and  b'.  Or  connect  bars  2  to  3  and  51  to  52 


110 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


and  disconnect  the  coil  between  bars  51  and  3  at  a"  and  6". 
For  definition  of  retrogressive  and  progressive  wave  winding, 
see  page  107,  Chapter  IV. 


94  95  9C  97 


45  4G  47  48  49 


93  94  95  9C  97 


FIG.  96a. — Method  of  cutting  out  a  damaged  coil  in  a  retrogressive  wave 

winding. 


t  2 


FIG.  966. — Method  of  cutting  out  a  damaged  coil  in  a  progressive  wave 

winding. 


345 


FIG.  97. — Double  layer  winding  for  a  4-pole  armature  having  24  slots  and 
24  commutator  bars.  The  connections  for  this  diagram  are  given  in  tabu- 
lated form  on  page  111. 

Tables  for  Placing  Coils  and  Connecting  Them  in  a  D.-C. 
Armature  Winding. — To  avoid  mistakes  in  reading  a  complete 
winding  diagram  and  to  avoid  the  necessity  of  making  such  a 
diagram,  Fig.  97,  it  is  advisable  to  furnish  an  inexperienced 
armature  winder  with  a  table  for  laying  the  coils  in  the  slots 
and  also  a  table  for  connecting  the  proper  leads  to  the  com- 
mutator. These  tables  can  be  made  up  as  follows: 


MAKING  CONNECTIONS  TO  THE  COMMUTATOR     111 
TABLE  FOR  PLACING  COILS  IN  ARMATURE  SLOTS — LAP  WINDING 


Coil  number 

Place  sides  of  coil  in 

Winding  space  numbers 

In  slot  numbers 

1 

1  and  12 

1  and  6 

2 

3  and  14 

2  and  7 

3 

5  and  16 

3  and  8 

4 

7  and  18 

4  and  9 

5 

9  and  20 

5  and  10 

6 

11  and  22 

6  and  11 

7 

13  and  24 

7  and  12 

8 

15  and  26 

8  and  13 

9 

17  and  28 

9  and  14 

10 

19  and  30 

10  and  15 

etc. 

etc. 

etc. 

24 

47  and  10 

24  and  5 

This  table  is  for  a  four-pole,  double  layer,  lap  winding  on  an 
armature  having  24  slots  and  24  commutator  bars.  By  giving 
the  throw  of  the  coils  both  in  winding  spaces  and  in  number  of 
slots  the  winder  will  not  become  confused  when  either  is  used 
in  referring  to  the  winding.  A  table  of  connections  to  the 
commutator  for  this  winding  is  made  up  as  follows: 

TABLE  FOR  CONNECTING  COIL  LEADS  TO  THE  COMMUTATOR — LAP 

WINDING 


Connect  terminals  of  coil 


tJoil  number 

Start  end  to  bar  number 

Finish  end  to  bar  number 

1 

1 

2 

2 

2 

3 

3 

3 

4 

4 

4 

5 

5 

5 

6 

6 

6 

7 

7 

7 

8 

8 

8 

9 

9 

9 

10. 

10 

10 

11 

etc. 

etc. 

etc. 

24 

24 

1 

112          ARMATURE  WINDING  AND  MOTOR  REPAIR 

Such  tables  are  particularly  useful  in  the  case  of  a  double 
layer  wave  winding  where  each  coil  is  wound  with  two  or  more 
wires  in-hand.  The  following  tables  for  a  four-pole,  double 
layer,  wave  winding  with  two  terminals  per  coil,  15  slots  and 
30  commutator  bars,  will  illustrate  their  usefulness  in  such  a 
winding: 


TABLE  FOR  PLACING  COILS  IN  A.RMATURE  SLOTS — DOUBLE  WAVE 

WINDING 


Place  sides  of  coil  in 


0011  numoer 

Winding  spaces  numbers 

In  slots  numbers 

1 

1  and  8 

1  and  4 

2 

3  and  10 

2  and  5 

3 

5  and  12 

3  and  6 

4 

7  and  14 

4  and  7 

5 

9  and  16 

5  and  8 

etc. 

etc. 

etc. 

16 

29  and  6 

15  and  3 

TABLE  FOR  CONNECTING  COIL  LEADS  TO  THE  COMMUTATOR — DOUBLE 
WAVE  WINDING 


Connect  terminals  of  coil 


Start  ends  to  bars  numbers 

Finish  ends  to  bars  numbers 

1 

1  and  2 

15  and  16 

2 

3  and  4 

17  and  18 

3 

5  and  6 

19  and  20 

4 

7  and  8 

21  and  22 

5 

9  and  10 

23  and  24 

etc. 

etc. 

etc. 

15 

29  and  30 

13  and  14 

Wave  vs.  Lap  Windings. — The  wave  winding  is  mostly 
used  on  the  smaller  types  of  direct  current  machines  of  multi- 
polar  construction.  This  winding  has  but  two  paths  through 
the  armature,  regardless  of  the  number  of  poles,  and  half  the 
coils  in  the  armature  are  connected  in  series  in  each  path, 


MAKING  CONNECTIONS  TO  THE  COMMUTATOR     113 

whereas  the  lap  winding  has  as  many  paths  as  there  are  poles, 
and  a  correspondingly  smaller  number  of  coils  in  series.  For 
instance,  in  the  case  of  a  four-pole  armature,  wound  with 
coils  of  the  same  number  of  turns  and  same  size  conductors; 
if  all  are  connected  in  a  wave  winding,  the  armature  will  be 
suitable  for  double  the  voltage  at  half  the  current  that  would 
be  used  if  the  same  armature  were  connected  as  a  lap  winding. 
Thus  on  small  machines,  where  the  number  of  turns  is  neces- 
sarily limited,  the  wave  winding  is  usually  much  cheaper  and 
easier  to  use.  The  wave  winding  is  widely  used  in  armatures 
for  railway,  hoist  and  crane  motors. 

The  limitation  in  the  use  of  the  wave  or  series  winding  is  in 
the  main  the  amount  of  current  which  can  be  carried  by  one 
armature  circuit.  Since  the  wave  winding  consists  of  two 
circuits,  the  current  per  circuit  is  equal  to  one-half  the  total 
armature  circuit  of  the  machine.  Where  the  value  of  this 
current  exceeds  that  which  has  been  found  to  be  consistent 
with  good  practice,  (up  to  250  amperes  per  circuit  in  non- 
interpole  machines  and  up  to  550  amperes  in  interpole  ma- 
chines) then  it  becomes  necessary  to  arrange  more  circuits  on 
the  armature,  each  circuit  carrying  a  part  of  the  total  current. 
In  this  case  the  lap  or  multiple  winding  is  usually  employed, 
which  as  explained  has  a  number  of  circuits  equal  to  the  number 
of  poles  of  the  machine.  However,  the  choice  of  wave  or  lap 
windings  is  usually  not  only  determined  from  the  amount  of 
current  to  be  handled  but  is  greatly  influenced  by  the  require- 
ments for  good  commutation  and  by  the  size  and  design  of  the 
armature  to  be  wound. 

It  is  customary  with  some  motor  manufacturers  to  use  wave 
windings  wherever  the  current  to  be  handled  permits  the  use 
of  a  conductor  of  a  sufficient  size  to  form  it  into  a  wave  shaped 
coil  and  where  the  number  of  coils  in  the  slot  does  not  become 
too  large.  The  latter  condition  is  objectionable  on  account  of 
the  space  required  for  insulating  the  coils.  Other  manufac- 
turers employ  the  lap  winding  wherever  possible,  that  is,  in 
cases  where  the  number  of  turns  per  slot  does  not  become 
too  small.  As  a  general  rule,  it  can  be  said  that  the  wave 
winding  often  works  out  to  advantage  in  cases  of  compara- 
tively large  machines  for  low  voltage  whereas  the  lap  winding 


114         ARMATURE  WINDING  AND  MOTOR  REPAIR 

is  preferable  in  cases  of  large  or  small  machines  for  high  vol- 
tage as  well  as  for  small  machines  for  low  voltage. 

Except  under  the  special  conditions  named  above,  such  as 
heavy  current  to  be  handled  by  the  armature,  machines  of 
high  voltage,  and  small  machines  of  low  voltage  where  the  lap 
winding  has  advantages  over  the  wave  winding,  there  is  little 
choice  between  the  wave  or  lap  winding  for  those  machines 
where  the  number  of  poles  may  equal  the  number  of  circuits 
of  which  the  winding  may  be  made  up.  It  may  be  said  that 
in  this  case,  American  practice  favors  the  lap  winding  and 
European  practice  the  wave  winding. 

It  might  be  pointed  out  further  that  in  a  wave  or  two-circuit 
winding  all  of  the  coils  between  two  diametrically  opposite 
points  on  the  winding  are  in  series  with  each  other  while  in  a 
lap  winding  only  the  coils  between  adjacent  pole  centers  are 
in  series.  Therefore,  under  similar  conditions,  the  voltage 
in  a  wave  winding  will  equal  the  voltage  of  a  lap  winding 
multiplied  by  the  number  of  pairs  of  poles.  The  voltage 
between  the  commutator  segments  will  also  vary  in  the  same 
ratio.  In  order  to  obtain  the  same  voltage  of  the  machine 
the  relative  number  of  coils  in  the  two  types  must  vary  in- 
versely with  the  number  of  poles  and  the  size  of  the  coils 
in  the  wave  winding  must  increase  in  proportion.  Where  the 
size  of  armature  coil  does  not  become  excessive  nor  the  voltage 
between  segments  too  great  to  permit  good  commutation, 
the  wave  winding  can  be  used.  These  conditions,  however, 
limit  this  type  of  winding  under  ordinary  conditions  to  four 
or  six  pole  machines.  In  cases  where  the  number  of  poles 
is  larger  the  lap  winding  has  a  wider  application  for  direct- 
current  armatures. 

In  regard  to  the  selection  of  the  proper  type  of  winding  for 
direct-current  and  alternating-current  machines,  Henry  Scheril, 
formerly  a  member  of  the  engineering  department  of  the 
Crocker- Wheeler  Company,  has  made  the  following  comment 
(Electrical  Record,  January,  1919) : 

The  selection  of  the  proper  kind  of  winding  depends  upon 
the  capacity  of  the  machine,  the  voltage  and  speed.  Wave 
windings  are  used  on  machines  of  small  capacity.  They  are 
also  being  used  for  low  speed  and  medium  ratings.  On  small 


MAKING  CONNECTIONS  TO  THE  COMMUTATOR     115 

high  voltage  machines,  like  the  railway  motor,  the  wave  wind- 
ing is  being  used  to  advantage  because  it  has  only  two  sets  of 
brushes  and  it  can  be  inspected  very  easily.  Wave  windings 
are  also  used  for  the  rotors  employed  in  induction  motors.  In 
all  other  cases,  lap  windings  are  to  be  preferred. 

Lap  Windings  for  Direct -current  Armatures. — In  general, 
the  ends  of  a  coil  of  lap  winding  in  a  direct-current  machine 
are  connected  to  the  adjacent  commutator  bars.  The  sides 
of  the  coil  arc  spread  over  and  placed  in  the  slots  of  the  arma- 
ture corresponding  to  the  distance  equivalent  to  180  electrical 


FIG.  98. — Lap  or  parallel  winding  for  a  6-pole  machine. 


degrees.  That  is  to  say,  if  one  side  of  the  coil  is  under  the 
center  of  a  north  pole,  then  the  other  side  of  the  coil  will  be 
placed  in  a  slot  having  a  similar  position  under  the  adjacent 
south  pole.  This  brings  one  to  a  consideration  of  the  wind- 
ing pitch.  The  pitch  is  the  number  of  slots  spread  over  the 
periphery  of  the  armature  corresponding  to  the  arc  equal  to  the 
distance  between  two  similar  points  of  two  consecutive  poles. 


116         ARMATURE  WINDING  AND  MOTOR  REPAIR 

For  instance,  if  a  machine  has  72  slots  and  eight  poles,  the 
full  pitch  will  be  72  -5-  8  =  9,  and  the  coil  will  lie  in  slots  of 
one  and  ten. 

In  a  direct-current  machine  the  lap  or  multiple  winding  has  a 
number  of  paths  for  the  current  to  travel  through  the  winding 
from  positive  to  negative,  equal  to  the  number  of  poles.  It 
becomes  necessary,  sometimes,  to  wind  a  machine  with  what 
is  known  as  the  "short  chord"  or  fractional  pitch  winding. 
In  this  kind  of  winding,  using  a  fractional  pitch,  the  coil  is 
spread  over  the  distance  which  takes  in  less  than  the  number 
of  slots  between  two  similar  points  on  two  consecutive  poles. 
The  use  of  this  kind  of  winding  on  a  direct-current  machine 
is  made  when  there  is  a  desire  to  improve  the  commutation. 
That  is,  because  with  a  fractional  pitch  winding  the  emf., 
due  to  inductance  of  the  armature,  is  lessened  at  the  time  of 
commutation.  In  direct-current  machines  it  is  not  advisable 
to  use  a  shorter  pitch  than  about  90  per  cent,  of  the  full  pitch. 
If  this  figure  is  exceeded  then  the  advantages  which  this  wind- 
ing would  give  are  practically  eliminated  and  liable  to  bring 
about  worse  results. 

All  interpole  machines  are  built  with  lap  windings  and 
equalizer  bars  are  used.  The  setting  of  the  brushes  on  inter- 
pole  machines  must  be  done  very  accurately  because  their 
position  is  fixed  once  and  for  all,  but  due  to  irregularities  in 
manufacture,  in  order  to  eliminate  any  possibility  of  poor  com- 
mutation equalizer  connectors  are  being  used. 

Lap  Windings  for  Alternating-current  Machines. — Practi- 
cally all  alternating-current  machines  use  windings  having  a 
short  pitch.  The  reasons  for  using  the  short  pitch  winding, 
especially  on  alternating-current  machinery,  are  many.  To 
begin  with,  most  manufacturers  standardize  their  frames, 
standardize  their  shields,  standardize  the  clearances  between 
the  rotary  elements  and  the  stationary  elements.  In  some 
machines,  the  clearances  between  the  windings  and  the  shields, 
especially  in  machines  carrying  heavy  currents,  and  also  in 
high  voltage  machines,  become  very  small  due  to  the  form  of 
end  connections  which  are  being  used  in  lap  windings.  It 
has  been  found  that  by  using  the  short  pitch  winding,  the 
end  connections  can  be  shortened  considerably  and  thus  ob- 


MAKING  CONNECTIONS  TO  THE  COMMUTATOR     117 

tain  more  clearance.  Another  reason  for  using  the  short 
pitch  winding  in  alternating-current  machinery  is  because  if 
the  machine  is  to  be  designed  with  existing  punchings,  there 
is  a  possibility  that  in  designing  the  winding,  one  may  get 
too  many  turns  by  using  the  full  pitch  due  to  a  greater  number 
of  slots  than  required.  By  reducing  the  pitch  from  full  to 
fractional,  the  same  results  are  brought  about  as  when  the 
number  of  slots  is  reduced.  In  alternating-current  machinery, 
short  pitches  are  used  as  low  as  %  of  the  full  pitch.  Practic- 
ally all  two-speed  induction  motors  have  %  pitch. 

Wave     Windings     for     Direct -current     Armatures. — The 
series  or  wave  winding  consists  of  coils  spread  over  the  per- 


P          N          1 

FIG.  99. — Wave  or  two  circuit  winding  for  a  6-pole  machine. 


iphery  of  the  armature  in  the  same  way  as  the  multiple  winding 
but  with  the  ends  of  the  coil  in  the  direct-current  machines 
connected  to  the  commutator  bars  whose  relative  positions 
correspond  to  about  double  pitch.  From  the  nature  of  this 
winding,  there  are  only  two  paths  for  the  current  to  flow  from 


118         ARMATURE  WINDING  AND  MOTOR  REPAIR 

positive  to  negative  brush  independent  of  the  number  of  poles. 
Now  since  the  winding  goes  around  the  armature  several 
times,  depending  upon  the  number  of  slots,  in  order  that  the 
winding  may  not  close  upon  itself,  the  number  of  coils  in 
series  must  be  one  more  than,  or  one  less  than  the  number  of 
poles.  Since  this  winding  has  only  two  circuits,  regardless  of 
the  number  of  poles,  only  two  sets  of  brushes  are  needed. 
However,  when  this  winding  is  being  used  on  machines  of 
large  capacity,  there  will  usually  be  as  many  sets  of  brushes 
as  there  are  poles.  The  purpose  of  this  is  twofold.  One  is 
to  reduce  the  size  of  the  commutator  and  the  other  is  to  reduce 
the  brush  density. 

Wave  Windings  for  Alternating -current  Machines. — In 
alternating-current  machines  wave  windings  are  mostly  used 
in  rotors  of  slip  ring  induction  motors.  They  are  either 
single  or  double  circuit  and  their  use  for  that  purpose  is  two- 
fold. One  reason  is  because  the  end  connections  are  much 
easier  to  make  than  in  the  case  of  the  lap  winding  and  the 
second  reason  is  because  of  certain  definite  ratios  to  be  ob- 
tained. Double  or  triple  wave  windings  are  also  used  on 
alternating-current  machines  of  very  low  voltage  and  high 
currents  where  lap  windings  are  found  to  be  not  practical 
because  of  the  form  of  end  connections.  All  wave  windings 
should  be  symmetrical. 

For  a  wave  winding  in  an  alternating-current  motor  the 
number  of  slots  (plus  or  minus  one)  is  chosen  so  as  to  be  divis- 
ible by  the  number  of  poles.  If  the  number  of  slots  is  plus 
one,  the  winding  is  called  progressive,  since  after  traveling  once 
around  the  stator  it  returns  to  the  starting  slot  plus  one.  If 
it  is  minus  one,  it  is  called  retrogressive  since  the  circuit 
returns  the  winding  to  the  starting  slot  minus  one.  In  this 
winding  the  correspondingly  placed  conductors  under  adja- 
cent poles  are  connected  in  series  with  the  circuit  proceeding 
from  pole  to  pole  several  times  around  the  stator.  The  circuits 
are  then  interconnected  to  give  the  required  phase  relations. 

Single  vs  a  Number  of  Independent  Windings. — Most 
machines  are  built  with  a  single  winding  with  the  exception  of 
some  three-wire  generators,  having  double  windings,  the  two 
windings  being  independent  of  each  other.  All  alternating- 


MAKING  CONNECTIONS  TO  THE  COMMUTATOR     119 

current  machines  are  built  with  windings  so  that  they  can  be 
easily  changed  from  multiple  circuit  to  single  circuit.  That  is 
to  say,  the  design  of  a  winding  for  an  alternating-current  ma- 
chine is  made  such  that  if  it  is  built  in  two  circuits,  for  say 
220  volts,  that  same  machine  can  have  the  windings  recon- 
nected for  440  volts. 

Double  or  triple  windings,  either  lap  or  wave,  have  been 
found  to  give  much  trouble  in  direct-current  machines,  espe- 
cially in  commutation.  Sparking  at  the  commutator  be- 
comes very  pronounced  and  the  commutator  wears  down  very 
rapidly.  For  this  reason  this  kind  of  winding  is  not  widely 
used  on  direct-current  machines. 

Lap  Windings  vs  Multiple  Wave  Windings. — Although  they 
have  not  been  much  used  in  the  past,  multiple  wave  windings, 
sometimes  called  series-parallel  windings  since  they  are  wave 
windings  with  more  than  two  circuits,  have  some  advantages. 
The  points  in  favor  of  their  use  are  outlined  as  follows  by 
Albert  A.  Nims  of  the  engineering  department  of  the  Crocker- 
Wheeler  Company  (Electrical  Record,  February,  1919) : 

There  seems  to  be  no  reason  for  avoiding  wave  windings 
with  more  than  two  circuits  on  ratings  where  they  would  be 
desirable,  provided  they  be  made  perfectly  symmetrical. 
This  condition  would  bar  out  a  four  circuit  winding  on  a  six- 
pole  machine.  Arnold,  who  is  generally  regarded  as  the  origi- 
nator of  this  type  of  winding,  did  not  always  insist  on  complete 
symmetry,  but  the  writer  believes  that  the  unsuccessful 
experiences  with  series-parallel  windings  in  the  past  has  been 
largely  due  to  an  incomplete  understanding  or  observance  of 
the  laws  of  symmetry  of  armature  windings. 

In  windings  with  circuits  equal  in  number  to  the  poles  mul- 
tiple or  lap  windings  are  ordinarily  used.  They  possess  the 
advantage  of  having  each  coil  commutated  at  only  one  brush, 
and  also  the  disadvantage  of  having  each  circuit  under  a  dif- 
ferent pole,  or  pair  of  poles.  If  the  field  strengths  under  the 
various  poles  are  not  identical,  the  induced  emf  s  in  the  various 
circuits  are  not  the  same.  Undesirable  currents,  which  may 
be  of  considerable  magnitude  because  of  the  low  resistance  of 
the  winding,  then  circulate  between  the  different  circuits, 
overloading  some  brushes  and  causing  them  to  spark,  and 


120          ARMATURE  WINDING  AND  MOTOR  REPAIR 

uselessly  heating  the  armature.  The  brushes  may  be  pro- 
tected by  diverting  these  currents  through  internal  equi- 
potential  connections,  but  the  useless  heating  of  the  armature 
still  remains.  This,  however,  is  usually  not  excessive  com- 
pared with  the  heating  of  other  losses,  so  that  this  winding  may 
be  called  standard  today. 

Series-parallel  wave  windings  with  circuits  equal  in  number 
to  the  poles  possess  the  disadvantage,  common  to  all  series 
wave  windings,  that  the  terminals  of  each  coil  lie  under  two 
different  brushes,  which  are  of  the  same  polarity  and,  therefore, 
connected  by  a  low  resistance  conductor.  That  is,  a  coil  is 
commutated  by  two  brushes  and  the  connector  between  them 
and  there  is  a  tendency  for  selective  commutation  or  unequal 
division  of  the  current  among  the  brushes  to  occur,  especially 
in  large  non-interpole  machines.  This  generally  results  in 
sparking  at  some  of  the  brushes.  They  are  eaten  away  so 
that  the  effective  position  of  their  contact  surfaces  are  changed. 
Other  brushes  then  get  more  current  than  they  should  and 
eventually  all  the  brushes  are  damaged.  Series-parallel 
wave  windings  do  have  the  advantage  that  each  circuit  is 
influenced  by  all  the  poles,  so  that  the  induced  emfs  in  all 
circuits  are  equal  and  there  is  little  tendency  for  circulating 
currents  to  occur  due  to  unbalanced  induced  emfs.  To 
eliminate  the  disadvantages  and  combine  the  advantages  of 
these  two  classes  of  windings,  multiple  wave  windings  have 
been  used,  particularly  on  four-pole  machines.  Although  they 
are  "  special"  and,  therefore,  cost  more  than  standard  windings, 
they  accomplish  their  purpose  admirably.  There  seems  to  be 
no  reason  why  they  would  not  be  equally  successful  on  ma- 
chines with  six  or  more  poles. 

Use  of  Equalizer  Rings. — Equalizer  rings  are  being  used  on 
lap  windings  only  on  large  multipolar  machines,  where  un- 
balanced conditions  in  the  magnetic  circuit  are  liable  to  cause 
circulating  currents  between  the  several  paths  through  the 
armature.  This  condition  will  occur  for  instance  in  the 
course  of  operation  where  the  air  gap  will  not  be  uniform 
so  that  the  emf  induced  in  a  coil  opposite  a  small  gap  will 
be  larger  than  that  induced  in  the  coil  which  is  opposite  the 
large  gap  and  this  difference  in  the  emfs  will  bring  about 


MAKING  CONNECTIONS  TO  THE  COMMUTATOR     121 

a  circulating  current  which  will  flow  between  the  windings  and 
thus  interfere  with  the  performance  of  the  machine.  This 
condition  will  also  occur  if  the  machine  has  been  in  operation 
any  length  of  time,  the  inequality  of  the  air  gap  being  brought 
about  by  the  wear  in  the  bearings.  The  same  condition  will 
occur  if  the  brushes  are  not  spaced  properly.  In  a  wave 
winding,  each  circuit  has  its  conductors  pass  under  all  poles 
and,  therefore,  there  is  no  necessity  of  using  equalizer  rings. 


CHAPTER  V 
TESTING  D.-C.  ARMATURE  WINDINGS 

The  common  causes  of  trouble  in  armatures  are  practically 
the  same  as  in  any  electrical  circuit,  namely,  a  short  circuit 
in  or  between  coils,  an  open  circuit,  reversed  coils  and  grounds. 
Of  all  the  faults  inherent  to  armatures,  probably  the  most 
dangerous  is  the  short  circuit  between  coils.  If  it  is  not 
detected  and  remedied  as  speedily  as  possible,  the  result  in 
most  cases  is  the  burning  out  of  the  coils  affected,  and  possibly 
the  whole  armature. 

Causes  of  Short  Circuit  in  an  Armature. — There  are  numer- 
ous ways  in  which  a  short  circuit  of  coils  may  occur.  In  the 
case  of  wire-wound  coils,  it  sometimes  happens  that  one  of  the 
turns  forming  the  coil  becomes  twisted  during  the  process 
of  inserting  the  coil  in  the  slot,  and  in  order  to  force  the  wind- 
ing down  to  an  even  depth,  the  turn  of  wire  was  driven  down 
upon  other  turns,  cutting  through  the  insulation  and  causing 
a  short  circuit  between  turns  of  the  same  coil.  When  this 
occurs,  the  resistance  of  the  coil  is  reduced,  allowing  more 
current  to  flow,  increasing  the  temperature  of  the  coil  and 
eventually  causing  a  deterioration  of  insulation  on  other  wires 
at  that  point.  In  the  majority  of  cases  this  results  in  short- 
circuiting  the  entire  coil  upon  itself. 

Also  a  frequent  cause  of  trouble  is  the  short  circuit  between 
coils  This  is  often  caused  on  the  back  end  of  the  armature 
by  oil  soaking  into  the  coils  by  leakage  from  the  out-board 
bearing,  which,  together  with  the  dust  that  will  invariably 
work  in  between  the  windings,  break  down  the  insulation  and 
cause  an  electrical  leak  between  coils.  A  short  circuit  may 
also  result  from  the  top  and  bottom  armature  leads  coming 
in  contact  with  each  other.  A  short  circuit  between  commuta- 
tor bars  is  often  the  cause  of  burned  out  coils.  In  soldering 
leads  to  the  commutator,  great  care  must  be  exercised  not  to 

122 


TESTING  D.-C.  ARMATURE  WINDINGS  123 

allow  any  of  the  molten  solder  to  run  down  behind  the  bars, 
as  this  very  frequently  short  circuits  the  bars.  A  good  way 
to  avoid  this  is  to  raise  the  back  end  of  the  armature  a  trifle 
so  as  to  allow  the  solder  to  run  to  the  front  where  it  can  be 
easily  removed. 

Tests  for  a  Short  Circuit  in  an  Armature. — Probably  the 
best  way  of  detecting  the  presence  of  a  short  circuit  in  the 
armature  of  a  motor  in  operation,  is  to  carefully  watch  it 
when  starting  as  soon  as  faulty  operation  is  noticed.  Some- 
times the  armature  will  not  start  upon  the  first  few  points 
of  the  rheostat,  and  will  then  take  an  excessive  current.  This 
will  cause  it  to  run  with  a  slow  and  unsteady  motion  (especially 
at  low  speeds),  due  to  the  fact  that  every  time  the  short-cir- 
cuited coil  comes  under  the  influence  of  a  pole,  it  will  have  a 
tendency  to  retard  the  motion  of  the  armature.  By  running 
the  motor  for  a  short  time,  the  bad  coil  will  heat  much  more 
than  the  others,  and  its  location  can  usually  be  detected  by 
passing  the  hand  over  the  end  windings. 

When  a  short  circuit  is  suspected  in  an  armature,  the  ma- 
chine should  be  shut  down  at  once  and  the  necessary  repairs 
made.  One  sure  method  of  locating  a  short-circuited  coil 
is  to  disconnect  all  the  leads  from  the  commutator  and  test 
out  the  coils  with  a  test  lamp.  A  test  lamp  consists  of  two 
wires  about  10  feet  long,  connected  to  a  110-volt  circuit  with 
an  incandescent  lamp  connected  in  series  in  the  circuit.  This 
method  requires  a  great  deal  of  work  in  unsoldering  all  the 
leads,  which  may  not  be  necessary  since  in  the  majority  of 
cases,  the  seat  of  the  trouble  will  be  found  in  the  commutator 
itself,  due  to  short  circuits  between  bars. 

A  rapid  test  often  used  to  locate  the  trouble  without  dis- 
connecting any  of  the  wires  on  the  commutator  during  the 
test,  is  the  bar  to  bar  test.  This  test  can  be  applied  to  arma- 
tures with  any  style  of  winding  connections,  for  there  will  be 
exactly  the  same  drop  of  potential  between  any  two  adjacent 
commutator  segments  no  matter  which  scheme  of  connection 
is  used.  Fig.  100  shows  the  connections  necessary  for  a  test  of 
this  kind.  A  steady  current,  taken  from  a  110-volt  circuit 
should  be  sent  through  the  armature  at  opposite  sides  of  the 
commutator.  The  brushes  B  should  only  be  wide  enough  to 


124 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


cover  one  bar.  C  is  a  fiber  block  holding  the  copper  contact 
points,  so  spaced  as  to  rest  on  adjoining  segments  as  shown. 
Adjust  the  lamp  bank  until  the  voltmeter  gives  a  readable 
deflection  when  C  is  in  contact  with  what  are  supposed  to  be 
good  coils-  The  deflection  of  the  voltmeter  will  depend  upon 
the  difference  of  potential  between  the  bars.  If  everything 
is  all  right,  practically  the  same  deflection  will  be  obtained 
all  around  the  commutator  regardless  of  what  pair  of  bars  C 


Mail 


1 


FIG.   100. — Connections  for  testing  out  armature  coils  with  a  millivoltmeter. 

may  rest  upon.  Pass  the  contact  points  over  each  pair  of 
bars  and  note  the  deflection  on  the  voltmeter.  When  the 
short-circuited  coil,  to  which  the  bars  are  connected,  comes 
under  the  contacts  there  will  be  very  little  if  any  movement  of 
the  needle,  because  there  will  be  little  or  no  drop  through  the 
coil  A  more  satisfactory  test  for  use  on  a  removed  armature 
is  the  transformer  test  discribed  on  page  125. 

When  the  coil  at  fault  has  been  found  in  this  manner,  its 
leads  should  be  disconnected  from  the  commutator,  together 
with  the  leads  adjoining  it  on  either  side.  The  commutator 
should  now  be  tested  by  use  of  the  test  lamp  to  determine  if 
the  bars  to  which  the  coil  was  connected  are  short-circuited. 


TESTING  D.-C.  ARMATURE  WINDINGS  125 

The  banding  wires  should  be  removed  from  the  armature 
next,  and  the  defective  coil  taken  out  by  raising  the  top  sides 
of  other  coils  clear  of  the  armature  as  far  around  as  the  bottom 
side  of  the  damaged  coil,  when  it  can  be  lifted  out.  In  many 
cases  the  insulation  on  the  wire  of  the  coil  has  reached  such  a 
stage  of  deterioration  that  a  new  coil  will  be  necessary,  in 
which  event  a  new  one  should  be  formed  with  exactly  the 
same  number  of  turns  and  size  of  wire  as  the  old  one.  Care 
should  be  taken  not  to  wrap  a  thicker  layer  of  tape  on  the 
new  coil  than  was  on  the  old  one,  for  if  this  is  done  trouble 
will  be  experienced  in  forcing  the  coil  back  into  the  slot.  The 
finished  coil  should  be  given  a  coat  of  insulating  varnish.  It  is 
a  good  plan  to  re-insulate  the  armature  slots  also,  before  re- 
turning the  coil.  If  the  commutator  is  free  from  short-cir- 
cuits, the  coil  may  be  replaced,  the  raised  coils  returned  to 
their  slots,  and  the  leads  soldered  to  the  commutator  again. 

Testing  for  Short  Circuits  and  Open  Circuits  with  a  Small 
Transformer. — While  the  method  of  testing  between  adjacent 
commutator  bars  with  a  millivoltmeter  will  indicate  short-cir- 
cuited or  poorly  soldered  leads  by  a  low  reading  and  open- 
circuited  or  poorly  soldered  leads  by  a  high  one,  it  often  occurs 
that  an  armature  is  rewound  and  reinstalled  with  considerable 
time  and  labor  and  found  to  be  defective  after  all.  The 
millivoltmeter  or  drop  of  voltage  method  merely  measures 
the  resistance  of  each  coil  but  when  an  armature  is  subjected 
to  magnetic  induction,  an  emf  is  induced  in  its  windings 
which  will  cause  current  to  flow  in  the  turns  that  are  short- 
circuited.  When  one  turn  in  a  coil  has  been  forced  so  hard 
against  another  that  the  insulation  is  broken  and  the  turns 
become  short  circuited,  the  millivoltmeter  test  may  not  serve 
to  detect  the  short  circuit  and  as  a  consequence  the  turns,  and 
probably  the  coil,  would  be  destroyed  by  the  immense  current 
which  would  flow  in  the  short-circuited  turns  when  running 
in  the  magnetic  field  of  the  machine. 

A  simple  way  of  detecting  such  defects  is  by  the  use  of  a 
small  transformer  often  called  a  "mill"  or  "bug"  and  con- 
structed as  shown  in  Fig.  101.  When  this  transformer  is 
applied  to  the  armature  core  as  shown,  the  alternating  flux 
produced  by  it  flows  through  the  core  and  produces  an  emf.  in 


126         ARMATURE  WINDING  AND  MOTOR  REPAIR 

the  coils.  If  the  winding  is  correct  no  current  will  flow  as 
the  voltages  will  balance  each  other.  If,  however,  a  coil  is 
short-circuited,  a  current  will  flow  in  the  turns  that  are  short- 
circuited.  To  locate  the  defective  coils  take  a  sharp  piece  of 
steel  or  a  knife  blade  and  pass  it  around  the  commutator  so  as 
to  short  circuit  in  succession,  the  coils  which  have  one  side 
under  the  transformer.  A  decided  sparking,  indicating  a 
potential  difference  between  the  bars,  shows  that  the  coil  is 
in  good  condition.  Absence  of  sparking  indicates  either 


LI 


FIG.   101. — Method  of  testing  an  armatuie  for  short  circuits  and  open  circuits 
in  coils  by  use  of  the  special  transformer  shown. 


an  open  circuit  or  a  short  circuit.  The  latter  can  be  readily 
determined  by  running  a  light  piece  of  sheet  iron  over  the 
surface  of  the  armature  core  so  as  to  bridge  the  slots  in  suc- 
cession. If  there  is  a  short  circuit  in  one  of  the  coils  which 
has  one  side  under  the  transformer,  a  local  current  will  flow 
through  this  coil  generating  a  magnetic  flux  which  will  attract 
the  piece  of  sheet  iron.  If  it  is  held  away  slightly  it  will  be 
made  to  vibrate  very  rapidly.  The  coil  will  also  heat  very 
rapidly  and  if  the  transformer  is  large  enough  for  the  armature 
being  tested,  the  coil  will  be  burned  out  completely. 

In  case  there  is  no  sparking  at  the  commutator  when  the 
coils  are  short-circuited  as  described  above  and  there  is  no 


TESTING  D.-C.  ARMATURE  WINDINGS  127 

local  magnetic  flux  when  moving  the  piece  of  sheet  iron  over 
the  slots  of  the  core,  an  open  circuit  is  indicated. 

A  transformer  cf  the  dimensions  shown  in  the  accompanying 
illustration  can  be  operated  on  a  110- volt,  60-cycle  circuit  and 
will  serve  for  testing  many  sizes  of  armatures.  The  one 
illustrated  was  made  up  by  E.  W.  Copeland  (Electrical  World) 
using  60  turns  of  No.  6  magnet  wire.  When  using  such  a 
transformer  it  should  be  fastened  under  the  armature  so  that 
there  is  just  enough  clearance  between  the  transformer  and 
armature  core  that  neither  will  touch.  Current  should  always 
be  off  while  placing  the  transformer  and  also  when  it  is  not 
in  use. 

When  testing  an  armature  with  a  small  exploring  trans- 
former, the  number  of  poles  in  the  machine  must  be  taken 
into  consideration.  In  a  two-pole  armature,  a  single  short 
circuit  may  heat  up  two  coils,  in  a  four-pole  armature,  four 
coils  and  so  on.  A  good  way  to  locate  the  defective  coil  is 
suggested  by  Maurice  S.  Clement  (Electrical  Record,  Novem- 
ber, 1918)  by  applying  the  telephone  receiver  test.  In  apply- 
ing this  test  the  terminals  of  a  telephone  receiver  are  placed 
on  adjacent  commutator  bars  which  are  connected  to  one  of 
the  affected  coils,  and  the  volume  of  sound  transmitted  to  the 
receiver  noted.  The  same  should  be  done  to  all  the  other 
affected  coils.  A  short  circuit  will  have  a  greater  volume  of 
sound  than  a  perfect  coil. 

Causes  of  Open  Circuits  in  an  Armature. — An  open  circuit 
may  result  from  a  number  of  causes.  In  the  first  place,  when 
the  armature  was  wound,  the  coil  may  have  been  driven  into 
position  in  such  a  manner  that  one  of  the  wires  was  strained 
or  partly  cut  in  two.  The  momentum  of  the  armature,  and 
constant  vibration  of  the  machine  will  finally  break  the  wire, 
and  in  this  way  form  an  open  circuit.  Sometimes  an  open 
circuit  of  this  kind  will  only  show  up  when  the  armature  is  up 
to  speed,  the  centrifugal  force  causing  the  wires  to  separate, 
thus  opening  the  circuit.  When  the  motor  is  at  rest,  the  wires 
will  come  together  again,  and  a  test  will  reveal  nothing.  This 
condition  is  known  as  a  " flying"  open  circuit,  and  occurs 
quite  frequently.  The  same  state  of  affairs  may  result  with 
a  short  circuit  between  overlapping  coils.  An  open  circuit 


128         ARMATURE  WINDING  AND  MOTOR  REPAIR 

may  also  be  caused  by  the  armature  leads  being  drawn  too 
tightly  when  they  are  soldered  to  the  commutator.  This  will 
cause  a  break  due  to  expansion  and  contraction  of  the  wire 
from  the  constant  heating  and  cooling. 

Another  common  cause  of  open  circuits  is  poor  workman- 
ship when  the  leads  are  soldered  to  the  commutator.  If  the 
lugs  or  risers  are  not  perfectly  tinned  before  attempting  to 
solder  the  leads  into  them,  the  solder  will  not  take  hold  over 
the  entire  area,  and  a  lead  may  be  held  in  place  only  by  a 
thin  film  of  solder  on  the  outside  surface.  When  the  current 
through  the  armature  is  heavy,  the  contact  area  between  the 
riser  and  the  leads  may  not  be  sufficient  to  carry  the  necessary 
current  without  excessive  heating.  This  will  melt  out  what 
little  solder  there  is,  and  an  open  circuit  will  result.  Sometimes 
a  commutator  will  become  so  hot  from  excessive  brush  friction, 
resistance  drop,  overloads,  or  the  like,  that  it  may  throw 
solder,  and  cause  an  open  circuit. 

Tests  for  an  Open  Circuit  in  an  Armature. — The  symptoms 
of  an  open  circuit  are  often  very  prominent.  A  vicious  green- 
ish-purple spark  will  usually  appear  at  each  brush  as  the 
open-circuited  coil  passes  from  one  pole  to  the  next.  This 
spark  has  a  tendency  to  leap  out  from  the  brush  and  follow 
around  the  commutator  for  quite  a  distance.  The  bars  to 
which  this  coil  is  connected  will  be  found  to  be  burned  and 
roughened,  and  the  mica  insulation  between  eaten  out  to  a 
considerable  depth. 

In  a  lap  wound  armature  the  position  of  an  open-circuited 
coil  is  easily  located,  because  each  end  of  the  coil  is  connected 
to  adjoining  bars.  In  a  wave  winding  this  is  not  the  case. 
Each  end  of  a  coil  is  connected  to  a  bar  removed  a  certain 
distance  around  the  commutator  from  the  other,  depending  of 
course,  upon  the  number  of  poles  and  the  winding  pitch 
employed. 

If  an  open  circuit  exists  in  an  armature  for  any  length  of 
time,  the  burned  condition  of  the  commutator  bars  will  usually 
indicate  where  the  trouble  is  located.  Both  lap  and  wave 
wound  armatures  may  be  tested  for  open  circuit  by  a  testing 
transformer,  by  use  of  the  ordinary  test  lamp,  and  a  bar  to  bar 
test,  or  by  ringing  out  between  adjacent  bars  with  a  magneto. 


TESTING  D.-C.  ARMATURE  WINDINGS 


129 


If  the  test  lamp  is  used,  test  the  commutator  from  bar  to 
bar  and  note  the  brightness  of  the  lamp  on  each  pair  of  bars. 
When  the  bars  are  reached  to  which  the  open-circuited  coil  is 
connected,  the  light  will  dim  considerably,  and  may  go  out, 
depending  upon  the  resistance  of  the  winding.  When  more 
than  one  coil  is  open-circuited,  the  winding  will  be  divided  into 
two  or  more  sections,  and  the  test  lamp  will  only  light  when 
the  test  leads  are  in  connection  with  the  bars  in  each  section. 

The  testing  set  described  for  locating  short  circuits  (page 
124)  may  also  be  used  for  open  circuits.  Proceed  in  the  same 
manner  as  when  testing  for  short  circuits.  When  the  con- 
tact points  C  (Fig.  100)  are  connected  to  the  open-circuited 
coil  (indicated  at  D),  there  will  be  a  violent  throw  of  the  needle, 
because  the  voltmeter  will  then  be  connected  to  brushes  B 
through  the  intervening  coils.  When  C  is  moved  to  the  next 
segments,  there  will  again  be  no  deflection,  thus  locating  the 
break  definitely. 

If  an  open  circuit  results  from  a  lead  breaking  off  at  the 
commutator,  it  is  an  easy  matter  to  solder  it  back  again. 
When  the  break  occurs  within  the  coil  itself,  a  new  one  must  be 
substituted,  as  described  for  the  short-circuit  test. 


y. — y 


'I'' 


i     i     '    '     ' 


'  i  i 


i  i  M 


1  234  S 


B  B' 

FIG.  102. — The  illustration  at  the  left  shows  method  of  bridging  a  coil  of  a 
wave  winding.     That  at  the  right  for  cutting  out  a  coil  of  a  lap  winding. 

Cutting  out  Injured  Coils. — In  case  of  emergency,  the  bad 
coil  can  be  cut  out  of  circuit  and  the  commutator  bars  con- 
nected together  by  a  wire  large  enough  to  safely  carry  the 
current.  This  wire  should  be  well  insulated  from  the  other 
leads,  as  any  connection  with  them  would  constitute  a  short- 
circuit.  In  Fig  102  the  method  of  bridging  out  a  coil  in  a 


130         ARMATURE  WINDING  AND  MOTOR  REPAIR 

wave  winding  is  shown.  When  one  of  the  coils  is  short-cir- 
cuited as  shown  at  A,  the  top  side  of  the  coil  is  disconnected 
from  bar  B,  and  the  bottom  side  from  bar  B'  Jumpers 
should  be  soldered  in  as  shown  by  the  dotted  line.  The  ends 
of  the  coil  leads  can  then  be  cut  off  close  to  the  armature  core 
and  taped.  The  coil  should  be  cut  completely  in  two  at  X 
and  X',  and  the  ends  taped.  This  will  prevent  self-induced 
currents  from  being  generated  within  the  coil,  which  might 
cause  heating  and  injure  the  insulation  on  other  good  coils.  In 
Fig.  102  (at  right)  the  method  of  cutting  out  a  coil  in  a  lap 
winding  is  shown.  The  coil  is  open-circuited  at  A.  The  top 
side  of  this  coil  should  be  disconnected  from  bar  3,  and  the 
bottom  side  from  bar  4.  In  this  case  the  jumper  must  be  run 
from  bar  3  to  bar  4.  The  dead  coil  can  be  taped  up  the  same 
way  as  the  series  or  wave  coil  mentioned  above. 

One  coil  cut  out  of  an  armature  will  not  perceptibly  affect 
the  running  of  a  motor,  and  several  of  them  can  usually  be 
cut  out  with  safety,  providing  they  are  not  bunched  together. 
It  is  not  wise  to  cut  out  too  many  coils,  as  this  increases  the 
heating  and  speed  of  the  armature  and  lowers  the  efficiency  of 
the  machine. 

Causes  of  Grounds  in  an  Armature. — A  ground  occurs  when 
current  leaks  from  the  current  carrying  parts  of  the  armature 
into  those  parts  that  are  not  intended  to  carry  current.  A 
single  ground  will  have  little  effect  on  the  operation  of  a  motor, 
but  it  should  be  removed  as  soon  as  possible,  as  there  is  always 
danger  of  a  second  ground  coming  on  at  some  other  point, 
which  would  produce  the  same  effect  as  a  short  circuit.  When 
a  ground  occurs,  a  small  hole  will  be  found  burned  through  the 
insulation  and  into  the  iron  parts  of  the  armature.  Across 
this  carbonized  insulation,  current  will  pass.  Grounds  occur 
very  frequently  on  the  ends  of  the  armature  core  at  the  points 
where  the  coils  leave  the  core.  If  the  bend  has  been  too 
sharply  made,  or  has  been  hammered  too  hard,  the  sharp 
edge  of  the  core  will  cut  through  the  insulation.  To  avoid 
this,  the  slot  insulation  should  extend  at  least  one-quarter 
to  one-half  inch  past  the  end  of  the  core  on  each  end. 

Grounds  also  frequently  occur  in  the  commutator,  caused 
by  oil  creeping  up  on  the  mica  ring.  Combined  with  the 


TESTING  D.-C.  ARMATURE  WINDINGS  131 

copper  and  carbon  dust  from  the  commutator,  this  forms  a 
good  path  for  leakage  of  current. 

Tests  for  Grounds  in  an  Armature. — A  ground  can  usually 
be  located  by  using  the  test  lamp.  Disconnect  the  leads  from 
four  of  the  commutator  bars  on  one-quarter  of  the  circumfer- 
ence. This  will  determine  the  section  of  the  winding  in 
which  the  ground  is  located.  Raise  the  leads  of  the  defective 
section  out  of  the  commutator.  Place  one  wire  of  the  test 
lamp  on  the  shaft,  and  with  the  other,  test  each  coil  separately 
to  locate  the  ground.  Sometimes  the  trouble  may  not  b&  in 
the  armature,  but  may  be  caused  by  a  grounded  commutator. 
If  the  coil  is  at  fault,  it  should  be  removed  and  reinsulated. 

Reversed  Coils. — A  reversed  coil,  that  is,  one  with  the  leads 
to  the  commutator  reversed,  frequently  occurs.  A  practical 
way  of  locating  a  reversed  coil  is  to  pass  a  current  through 
the  armature  at  opposite  points.  The  lamp  bank  and  con- 
nections of  Fig.  100  can  be  used  for  this  test.  Then  with  a 
compass  or  small  bar  of  magnetized  steel  explore  around  the 
armature  to  determine  the  direction  of  magnetism  from  slot 
to  slot.  When  the  compass  is  over  the  reversed  coil,  the  needle 
will  reverse,  giving  a  very  definite  indication  of  the  coil  which 
is  connected  wrong.  The  leads  of  this  coil  should  simply  be 
reversed. 

Use  of  a  Bar  Magnet  and  Millivoltmeter  to  Locate  a 
Reversed  Armature  Coil. — In  armatures  where  both  leads  of  a 
coil  are  taped  together,  and  led  out  from  an  identical  point, 
there  is  considerable  danger  of  getting  the  leads  crossed  while 
connecting,  thus  reversing  the  direction  of  flow  of  the  current 
in  that  coil.  Such  a  reversal  will  in  all  cases  "light  out" 
as  though  perfect  when  tested  with  a  lamp,  but  will  cause 
bucking  when  the  machine  is  run.  A  reversed  coil  of  this 
sort  is  unusually  difficult  to  locate  and  in  many  cases  a  whole 
machine  has  been  stripped  because  ordinary  methods  of  testing 
failed  to  locate  the  trouble. 

An  efficient  test  which  has  been  used  by  J.  G.  Yoerns 
(Electrical  Record,  September,  1918)  is  illustrated  in  Fig.  103. 
It  will  be  noticed  that  a  millivoltmeter  is  used  rather  than  a 
voltmeter  because  of  the  greater  sensitiveness  of  the  former. 
Both  terminals  of  the  meter  are  connected  to  adjacent 


132 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


commutator  bars.  Next,  take  a  piece  of  metal  which  has 
been  magnetized  and  move  it  in  a  direction  corresponding 
to  the  revolving  of  the  armature,  directly  above  the  coil 
to  which  the  meter  is  connected.  It  is  well  to  keep  in  mind 
the  fact  that  if  a  clockwise  motion  is  used  on  the  first  coil,  the 
same  direction  must  be  retained  on  all  remaining  coils, 
otherwise  the  meter  reading  will  be  backward.  On  the 
downward  stroke  of  the  magnetized  bar  and  as  it  approaches 
the  coil  to  be  tested,  magnetic  lines  of  force  will  travel  from 
the 'bar  to  the  coil,  thence  to  the  meter,  causing  the  needle 
to  fluctuate  slightly.  If  the  needle  jumps  ahead  on  the 
downward  stroke,  it  will  jump  backward  on  the  upward 
stroke,  and  vice  versa.  The  reversed  coil  will  read  opposite 
from  the  others.  To  change  the  direction  of  the  fluctuations 
of  the  needle,  either  reverse  the  meter  terminals  or  reverse 
the  motion  of  the  magnet. 

iwrfvorfrnefer  Compass 


FIG.  103. — Method  of  using  a 
magnetized  bar  and  millivoltmeter 
to  locate  a  reversed  armature  coil. 


FIG.  104. — A  convenient  method 
for  using  a  compass  when  testing 
for  a  reversed  coil. 


Use  of  a  Compass  to  Locate  a  Reversed  Armature  Coil. — 

In  connecting  up  small  armatures  with  lap  windings  wound  on 
by  hand  with  four  or  more  leads  coming  out  of  each  slot, 
the  leads  may  be  easily  confused  as  already  mentioned  so  that 
some  of  the  individual  armature  coils  will  be  reversed.  E.  C. 
Parham  (Electrical  Record,  August,  1918)  has,  therefore, 
suggested  the  use  of  the  testing  device  shown  in  Fig.  104. 
While  several  reversed  coils  distributed  around  the  armature 


TESTING  D.-C.  ARMATURE  WINDINGS  133 

may  not  cause  sufficient  effect  to  excite  suspicion,  they  are 
likely  to  give  trouble  in  time.  It  is  conceivable  that  if  alter- 
nate coils  were  reversed,  a  highly  improbable  condition,  the 
armature  would  be  inoperable  because  then  there  would 
be  an  equal  number  of  coils  tending  to  turn  the  armature 
in  opposite  directions. 

The  accompanying  diagram  (Fig.  104)  shows  a  simple  cheap 
method  of  readily  locating  any  reversed  coils  that  may  exist. 
The  armature  rests  in  a  support  A  that  permit  of  rotating  the 
armature  as  the  test  progresses.  A  strip  of  copper  B  is  bent 
over  the  armature,  as  indicated,  and  a  compass  placed  upon  it. 
Current  from  an  incandescent  lamp  test  circuit  is  then  applied 
to  adjacent  commutator  bars  that  are  connected  to  the  coils 
that  lie  in  the  slot  that  is  immediately  under  the  compass. 
Suppose  that  the  compass  needle  is  deflected  to  the  right  on 
touching  bars  1  and  2  and  bars  2  and  3,  there  being  two  coils 
per  slot.  Rotate  the  armature  until  the  next  slot  comes  under 
the  compass  and  touch  the  test  points  to  bars  3  and  4  and  then 
to  bars  4  and  5,  and  so  on  all  round  the  commutator. 

The  compass  deflections  obtained  should  be  always  in  the 
same  direction.  Any  pair  of  adjacent  bars  touched  by  the 
test  points  causing  a  reversed  deflection,  includes  a  coil  the 
leads  of  which  have  been  brought  down  to  the  commutator 
in  reversed  order.  In  order  to  test  the  effectiveness  of  the 
method,  it  is  necessary  to  only  apply  the  test  points  to  adjacent 
commutator  bars  in  reversed  order  and  observe  that  the  com- 
pass deflection  is  thereby  reversed. 

Locating  Low  Resistance  or  Dead  Grounds. — It  is  often 
difficult  to  locate  a  low  resistance  or  "dead  ground"  in  a  low 
resistance  armature  owing  to  the  very  low  resistance  of  the 
windings  themselves.  In  such  cases  the  following  method 
can  be  used: 

First,  short  circuit  all  commutator  bars  by  winding  several 
turns  of  bare  copper  wire  around  them;  then  apply  a  source 
of  energy,  direct  current  being  preferable,  to  the  commutator 
and  shaft.  The  voltage  to  be  used  depends  upon  the  resistance 
of  the  "ground. "  This  produces  a  circuit  from  the  commuta- 
tor through  the  grounded  coil  to  the  ground  and  out  through 
the  shaft,  thus  setting  up  a  field  around  the  conductors  in 


134 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


this  coil.  By  applying  a  small  piece  of  iron  to  the  surface  of 
the  armature  core  and  gradually  moving  it  around,  the 
grounded  coil  can  be  located  by  means  of  its  field,  which  will 
attract  the  iron. 

The  same  method  can  also  be  applied  to  alternating  current 
windings  although  not  quite  so  readily.  For  example,  in 
the  case  of  a  three-phase,  single-circuit,  F-connected  armature, 
first  disconnect  the  F,  splitting  the  winding  up  into  three 
separate  circuits.  Then  test  out  each  circuit  with  a  magneto 


\ 

1 

i? 

1 

i  i* 

1 

i     -. 

\ 

1 

r 

y  j/° 

1    — 

1 

i 

TJ 

1 

d 

3 

I 
1 

2 

O  1 

J 

j 

/ 

/ 

W    Ground  to  Armature  Shaft 
FIG.  105. — Method  of  locating  grounds  in  an  armature  winding. 

or  test  lamp.  Next  apply  a  current  to  one  end  of  the  grounded 
circuit  and  to  the  shaft.  Assume  that  there  are  12  coils  as 
shown  in  Fig.  105,  coil  No.  7  being  the  grounded  coil  while 
coil  No.  1  is  connected  to  the  line  as  shown.  There  will  then 
be  a  circuit  through  coils  Nos.  1,  2,  3,  4,  5,  6,  and  7  which  can 
be  readily  detected  with  a  piece  of  iron  as  already  explained, 
while  coils  Nos.  8,  9,  10,  11,  and  12  are  dead.  It  is  then,  of 
course,  obvious  that  if  coils  Nos.  1,  2,  3,  4,  5,  6,  and  7  carry  a 
current  while  coils  Nos.  8,  9,  10,  11,  and  12  carry  no  current, 
the  ground  must  be  in  some  section  of  coil  No.  7,  the  circuit 
being  completed  at  that  point. 


TESTING  D.-C.  ARMATURE  WINDINGS 


135 


Use  of  a  Telephone  Receiver  in  Testing  for  Short  Circuits, 
Open  Circuits  and  Grounds  in  an  Armature. — The  telephone 
receiver  on  account  of  being  very  sensitive  to  sound,  makes 
a  convenient  testing  instrument  in  a  repair  shop.  The  ways 
it  can  be  employed  to  discover  a  short  circuit,  open  circuit, 
or  ground  are  given  here  as  used  by  Maurice  S.  Clement 
(Electrical  Record,  December,  1918). 

This  testing  device  can  be  used  either  with  alternating  cur- 
rent or  direct  current.  If  alternating  current  is  to  be  used,  a 
test  lamp  and  a  pair  of 
leads  from  a  110- volt  cir- 
cuit are  connected  to  the  Source  of  Energy 

commutator  as  in  Fig.  1C6, 
from  one-fourth  to  one-half 
of  the  circumference  apart. 
Next,  take  the  receiver 
which  has  about  two  feet 
of  two-wire  telephone  cord 

attached  and  hold  it  to  the         FlG     i06.-Connections    for    using    a 
With  the  Other  hand     telephone     receiver    for    locating    short- 
i       i       circuits  and  open  circuits  in  an  armature 


ear. 
press 


the 


recever 


winding. 

firmly  to  the  commutator, 

taking  care  to  touch  adjacent  segments.  Move  from  one  lead 
of  the  test  lamp  to  the  other  segment  by  segment  and  repeat 
the  operation  until  the  commutator  has  been  circled. 

If  the  wire  in  the  coils  is  of  too  low  a  resistance  to  make  a 
buzz  in  the  receiver,  put  a  rosette  fuse  in  place  of  the  test 
lamp  and  cut  a  rheostat  in  series  with  the  fuse.  This  will 
bring  the  resistance  up  sufficiently  to  make  a  buzz  in  the 
receiver.  A  low  buzz  indicates  a  good  flow  of  current;  if 
no  sound  whatever  can  be  heard  from  two  commutator  bars, 
there  is  a  dead  short  circuit.  An  open  circuit  is  indicated  by  a 
very  loud  buzz.  A  "cross  connection  will  produce  a  defective 
sound  on  three  segments.  These  three  leads  should  be  taken 
off  and  reconnected  immediately  and  the  receiver  test  once 
more  applied. 

For  a  dead  ground  which  persists  in  remaining  invisible, 
place  one  side  of  the  receiver  line  to  the  shaft,  as  in  Fig.  107, 
and  the  other  side  of  the  test  line  to  the  commutator,  then, 


136 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


Test  Lamp 
Source  of  Energy  j '^ 


with   the   receiver   to   the    ear,    buzz    each    segment.     The 

grounded  coil  will  buzz  louder  than  the  rest. 

If  direct  current  is  to  be  used,  the  source  of  energy  should  be 

a  battery.     A  buzzer  connected  in  series  on  one  side  of  the 

battery  completes  arrangements. 

Sometimes,  when  testing  an  armature  with  a  transformer, 

a  single  short  circuit  will  heat  up  coils  in  two,  four,  six  or  eight 

places,  according  to  the 
polarity  of  the  winding. 
By  applying  the  receiver 
leads  to  the  bars  con- 
nected to  each  coil  thus 
affected,  a  short  circuit 
can  be  located  quickly. 
Testing  for  Reversed 

Ground  on     -^  and  Dead  Field  Coils.— 

Machine  Frame  In   multipolar  machines 

FIG.   107  —Connections  for  using  a  tele-  tne  polarity  of  field  coils 

phone  receiver  for  locating  grounded  coils  J                 . 

in  an  armature  winding.  can     be     tested     with     a 

carbon  filament  lamp. 

By  placing  the  lamp  while  lighted  between  the  pole  tips,  the 
loops  of  the  filaments  will  draw  close  together  or  separate  far 
apart  depending  upon  the  direction  of  the  magnetic  flux.  A 
dead  pole  can  be  quickly  found  in  this  way. 

Another  very  simple  and  positive  method  for  testing  the 
polarity  of  field  coils  is  by  means  of  two  ordinary  iron  nails. 
To  do  this,  pass  a  current  through  the  field  coil  winding  and 
place  the  nails  on  two  adjacent  poles.  If  the  polarity  is  right, 
they  will  attract  each  other  and  if  wrong  they  will  repel  each 
other.  This  method  can  be  used  also  for  detecting  a  dead  coil. 

The  Commutator. — Trouble  in  a  commutator  may  be  traced 
to  either  a  short  circuit  or  a  ground.  In  a  commutator  a 
short  circuit  may  be  caused  by  any  of  the  following:  Particles 
of  metal  touching  two  adjacent  bars,  or  mica  burned  away 
allowing  current  to  arc  across.  In  an  undercut  commutator 
be  sure  the  slots  between  bars  are  thoroughly  free  from  all 
dirt  before  testing.  A  grounded  commutator  must  invariably 
be  removed  from  the  shaft,  and  the  damaged  part  well  cleaned 
and  reinsulated.  In  a  case  where  a  motor  has  been  through  a 


TESTING  D.-C.  ARMATURE  WINDINGS 


137 


138         ARMATURE  WINDING  AND  MOTOR  REPAIR 

fire,  the  segments  will  in  nearly  all  cases  be  undamaged,  but 
the  mica  will  be  burned  beyond  future  use,  and  in  such  a  case 
refilling  will  be  necessary. 

Testing  Equipment  for  a  Repair  Shop. — In  addition  to  the 
facilities  necessary  for  testing  out  armatures  and  machines 
for  short  circuits,  open  circuits,  grounds  and  the  like,  every 
repair  shop  must  be  equipped  to  test  and  load  motors  of  differ- 
ent sizes  requiring  voltages  from  110  to  2300.  A  voltage  of 
from  1200  to  1500  volts  is  needed  for  testing  an  armature  for 
grounds.  A  transformer  is  therefore  required  with  taps  and 
connections  so  that  the  voltage  can  be  varied  from  about  500 
volts  up  to  2500  volts. 

For  testing  for  short  circuits  and  open  circuits  a  small 
transformer  with  specially  shaped  laminations  about  which  a 
coil  of  wire  is  wound  should  be  available  (see  Fig.  101  on  page 
126).  When  applied  to  an  armature  with  a  short-circuited 
coil,  the  defective  coil  will  show  up  by  getting  hot. 

For  testing  out  circuits  for  opens,  and  to  match  up  ends  of 
coils  when  inserting  them  in  the  armature  core,  a  test  lamp  and 
terminals  is  convenient.  A  magneto  is  also  used  for  this 
purpose.  In  the  case  of  testing  for  open  circuits,  two  leads 
connected  to  a  110- volt  circuit  with  a  lamp  connected  in  series 
in  one  of  the  leads  is  sufficient.  For  selecting  coil  terminals 
or  "  lighting-out "  an  armature  winding  while  in  the  machine 
to  discover  trouble,  a  6-  or  12- volt  automobile  storage  battery 
provided  with  two  leads  having  snap  testing  clips  and  a  lamp 
in  series  with  one  lead,  is  a  convenient  outfit  that  can  be  taken 
about  the  shop  and  to  any  job. 

At  least  one  portable  ammeter  and  one  voltmeter  are  needed 
and  one  ringing  out  magneto  and  a  pair  of  head  telephone 
receivers. 

For  testing  motors  a  small  switchboard  wired  so  that  con- 
nections can  be  made  on  the  front  by  plugs  and  the  voltage 
increased  by  using  jumpers,  is  a  convenient  outfit  that  saves 
much  time  in  making  a  test.  This  switchboard  should  have 
mounted  on  it,  a  voltmeter,  ammeter,  frequency  meter  and 
the  necessary  switches  to  connect  and  operate  both  motors  and 
generators  on  either  direct  or  alternating  current. 


CHAPTER  VI 


OPERATIONS  BEFORE  AND  AFTER  WINDING 
D.-C.  ARMATURES 

Before  an  armature  goes  to  an  experienced  armature  winder 
and  after  it  leaves  his  hands,  there  are  certain  steps  in  the 
process  which  can  be  properly  termed  "before"  and  "after" 
operations.  In  a  case  where  a  damaged  armature  must  be 
Completely  rewound,  these  operations  may  be  outlined  as 
follows : 


Operations  before  winding 


Operations  after  winding 


Stripping  off  old  winding. 

Cleaning  slots  and  ends  of  core. 

Filing  burrs  off  slots. 

Testing  commutator. 

Repairing  commutator. 

Making  new  coils. 

Insulating  ends  of  core  and  slots. 


Testing  out  winding. 
Soldering  leads  to  commutator. 
Hooding  armature. 
Banding  armature. 
Turning  commutator. 
Undercutting  mica  of  commutator. 
Balancing  armature. 
Painting  armature. 
Relining  bearings. 


Reference  has  already  been  made  to  a  number  of  these 
operations  in  connection  with  the  procedure  in  rewinding 
machines  as  outlined  in  Chapters  III  and  VIII.  The  details 
that  are  given  here  refer  to  the  requirements  in  all  cases,  with 
references  to  other  Chapters  where  the  operation  has  been 
taken  up  as  a  special  subject. 

Stripping  Off  an  Old  Winding. — In  this  operation  when 
band  wires  must  be  removed,  they  should  be  cut  or  filed  in 
several  places.  When  a  chisel  and  hammer  are  used  care  must 
be  exercised  not  to  mash  down  the  armature  teeth.  The  next 
step  is  to  unsolder  the  leads  to  the  commutator  and  clean  out 
the  slits  hi  the  commutator  necks  carefully.  By  pulling  out 

139 


140         ARMATURE  WINDING  AND  MOTOR  REPAIR 


the  top  sides  of  the  proper  number  of  coils  according  to  the 
throw,  the  bottom  sides  can  be  reached  that  will  allow  the 
coils  to  be  removed  easily.  (See  Chapter  III.) 

Cleaning  and  Filing  Slots. — After  the  coils  have  been  re- 
moved, all  old  insulation  must  be  thoroughly  taken  off  the 
core  and  the  slots.  A  solution  composed  of  25  per  cent,  alco- 
hol and  75  per  cent,  benzole  is  good  for  loosening  the  varnish 
of  old  insulation  so  that  it  can  be  scraped  from  the  slots  in 
the  armature.  This  will  produce  no  bad  effects  on  the  lami- 
nations or  on  the  winding  when  the  armature  is  rewound. 
Alkali  solutions  such  as  caustic  soda  will  also  loosen  the  insula- 
tion without  injury  to  the  laminations,  but  will  creep  be- 
tween the  laminations  and  after  the  armature  is  rewound  the 
alkali  fumes  are  liable  to  damage  the  insulation.  A  tool 
made  from  bar  steel  one  by  one-sixteenth  inch,  of  suitable 
length  and  drawn  down  to  a  long  thin  point  like  a  chisel 
answers  very  well  to  remove  old  insulation  after  it  has  been 
softened,  and  will  also  give  very  satisfactory  results  with- 
out the  use  of  any  chemicals  whatever.  After  removing 
the  insulation  in  this  way  a  file  drawn  through  the  slots  will 

remove  small  pieces  of  insulation 
and  smooth  off  all  roughness.     The 
edges  of  the  slots  should  be  filed  to 
Xsiit8  for      remove  sharp  edges  and  burrs  that 
thread  to  tie    would  injure  the  new  coils  while 

coiltogether     ^.^      ^^      ^      ^      ^^         ^ 

entire   core  should  then  be  thor- 
with    a   blast   of 


-KJ 


oughlv    cleaned 

FIG.  109.— Shuttle  form  for 
winding  a  coil  that  can  be  pulled     compressed  air. 
into  a  diamond  shape  in  a  spe- 
cial pulling  machine  (Fig.  291). 


Testing  Commutator. — Details 
of  this  operation  are  given  in 
Chapter  XII  as  well  as  the  steps  in  repairing  a  commutator, 
testing  it  out  after  reassembly,  baking  the  insulation  and 
tightening  end  rings. 

Making  New  Coils. — This  operation  calls  for  an  inspection 
of  the  old  coils  and  winding  data  as  outlined  on  page  57, 
Chapter  III.  In  case  of  changes  in  speed  or  voltage  it  requires 
certain  calculations  which  involve  the  size  of  wire  to  use.  These 
calculations  are  given  in  Chapter  X,  page  240.  When  the 


OPERATIONS  BEFORE  AND  AFTER  WINDING        141 

* 

size  of  wire  is  known  the  winding  of  the  coils  can  be  done  on 

dne  of  the  forms  shown  in  Figs.  109  to  114. 

In  making  coils  in  large  repair  shops  three  methods  are 
used,  namely  winding  on  a  mould,  on  a  former  or  on  a  shuttle. 
Mould  coils  are  usually  those  made  -on  a  form  rotated  in  a 
lathe  with  all  necessary  shaping  done  with  very  little 
pounding  on  the  conductors.  By  formed  coils  is  usually 
meant,  those  coils  made  over  a  stationary  form  with  the  bends 


FIG.  110. — Three  steps  in  the  construction  of  shuttle  wound  coils  (Fairbanks- 
Morse  &  Company). 

(a)  The  coil  at  the  bottom  is  shown  as  wound  on  a  shuttle  form  using  square  copper 
wire.  (6)  At  the  left  it  is  shown  after  being  pulled  into  final  shape,  dipped  in  insulating 
varnish,  thoroughly  baked,  and  then  taped,  (c)  The  finished  coil  is  shown  at  the  right 
after  four  to  six  alternate  dipping  and  baking  treatments. 

made  by  the  use  of  levers  and  mallets  to  force  the  coil  to  the 
proper  shape.  Shuttle  or  "pulled"  coils  are  first  wound  on 
a  simple  shuttle  such  as  shown  in  Fig.  109  which  is  fastened 
to  a  lathe  and  then  pulled  on  a  coil  puller  to  the  shape 
required  for  the  particular  throw  of  the  coil  as  shown  in 
Fig.  110. 

Forms  for  Winding  Coils  Like  Those  Previously  Used. — 
A  simple  form  that  can  be  made  from  a  pine  board  for  dupli- 


142         ARMATURE  WINDING  AND  MOTOR  REPAIR 


eating  the  shape  of  coils  previously  used  in  an  armature  that 
is  being  rewound  is  shown  in  Fig.  111.  The  dimensions  and 
general  shape  can  be  secured  from  a  sample  coil  preserved 
from  the  old  winding.  In  the  illustration  A  is  a  small  hole 
for  mounting  on  a  spindle.  Small  pins  B  are  driven  into  the 
form  to  hold  the  coil  in  place  while  winding.  C  and  C1  are 
slots  cut  into  the  sides  so  that  the  several  turns  of  the  coil 
may  be  tied  together  with  thread.  This  keeps  the  wires 
together  after  the  coil  is  removed  from  the  form.  At  the  ends 
D  and  Df  two  pins  are  used  in  order  to  make  the  turn  shown 
on  the  end  section  at  the  right  in  Fig.  111. 


c' 

c 


I 


FIG.  111. — Form  for  shaping  diamond  armature  coils  to  match  old  ones  used. 

Now  mount  the  form  on  a  spindle  so  that  it  can  be  revolved 
and  wind  on  the  required  number  of  turns,  using  the  pins  as  a 
guide.  If  the  completed  coil  is  to  consist  of  two  coils,  take 
two  wires  and  wind  them  together.  When  the  proper  number 
of  turns  have  been  made,  cut  off  the  wire  and  tie  the  coil  with 
thread  at  C  and  C'.  Remove  the  coil  from  the  form  and  com- 
pare it  with  a  sample  if  such  a  sample  was  preserved  from  the 
old  windings.  It  is  also  well  to  try  this  first  coil  in  the  arma- 
ture, and  see  that  it  has  the  proper  span,  and  that  the  leads  to 
the  armature  are  long  enough. 

The  following  method  for  winding  coils  for  small  motors 
has  been  found  convenient  by  Maurice  S.  Clement  (Electrical 
Record,  November,  1918)  in  those  cases  where  a  repair  must 


OPERATIONS  BEFORE  AND  AFTER  WINDING        143 


be  made  on  the  job  or  coils  made  up  where  a  coil  winding 
machine  is  not  available.  The  form  is  made  up  as  follows: 
First  take  a  flat  piece  of  wood  about  three-fourths  of  an  inch 
thick  and  plot  out  the  shape  of  the  coil.  Then  place  right 
angle  screw  hooks,  at  all  angle  points.  The  screw  hooks 
should  point  away  from  the  center  of  the  form.  Mark  out  the 
center  and  run  a  breast  drill  through  as  far  as  it  will  go.  Place 
the  breast  drill  in  a  vise  in  a  horizontal  position  with  the  coil 
form  toward  the  left,  taking  care  to  place  it  so  as  to  allow  free 


Bolts  / 
extending 
through 
form 


FIG.  112.  FIG.  113. 

FIG.   112. — A  six  gang  form  for  winding  coils  in  series. 

Six  coils  can  be  wound  in  series  and  removed  in  a  group  so  they  can  be  inserted  in  the 
slots  of  an  alternating  current  motor  stator  without  the  necessity  of  makihg  the  series 
connections.  This  form  can  also  be  used  to  wind  a  single  or  double  coil.  It  is  built  up  in 
sections  which  are  held  together  by  the  bolts  at  A.  The  slots  in  the  divisions  between 
sections  enable  the  winding  and  removal  of  the  coils  in  series. 

FIG.  113. — Continuous  jointless  phase-group  coils  used  on  rotors  of 
Fairbanks- Morse  phase  wound  motors.  These  coils  are  wound  on  a  form 
similar  to  that  shown  in  Fig.  112. 

movement  of  the  handle.  Turn  the  wire  of  which  the  coil  is 
to  be  made,  once  or  twice  around  a  nail  on  the  back  of  coil 
form,  and  lead  the  wire  over  the  edge  to  the  face  of  the  form 
and  turn  the  handle.  The  form  can  be  revolved  at  any  de- 
sired rate  of  speed  and  the  wire  run  over  all  the  screw  hooks. 
Keep  sufficient  tension  on  the  wire  to  permit  each  turn  to  lie 
snugly  beside  its  predecessor. 

This  is  a  convenient  way  to  wind  coils,  for  the  reason  that 
unnecessary  crossing  of  wires  can  be  prevented.  When  the 
specified  number  of  turns  have  been  wound,  twist  a  short 
piece  of  wire  around  each  end  of  coil  to  hold  it  in  proper  shape. 


144 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


To  finish  the  operation,  turn  all  screw  hooks  toward  the  center 
of  the  form  and  slip  the  coil  off. 

Another  hand-made  form  also  recommended  by  Mr.  Cle- 
ment for  use  in  winding  larger  coils  than  the  one  described 
above  is  shown  in  Fig.  114. 

Insulation  of  Core  and  Slots. — The  insulation  needed  with 
coils  used  in  partially  closed  slots  and  in  open  slots  of  alternat  - 
ing  current  machines  is  given  in  Chapter  VIII  and  in  connection 
with  the  winding  of  direct-current  machines  in  Chapter  III. 


Slit  through  whicli 
First  Lead  of  Coil 
—  Passes 

Slits  through  which 
Strings  are  Laid 
to  Bind  Coil 


-Handle 


-Clamp 


Plate 


FIG.  114.  —  Construction  of  a  convenient  bench  winder  for  forming  armature 

coils. 

Testing  Out  the  Winding.  —  Details  of  this  operation  both 
before  the  leads  to  the  commutator  are  connected  and  when 
trying  to  locate  troubles  in  the  winding  are  given  in  Chapter  V. 

Soldering  Coil  Leads  to  the  Commutator.  —  After  the  wind- 
ing of  an  armature  has  been  thoroughly  tested  out,  the  coil 
terminals  can  be  soldered  to  the  commutator.  In  the  solder- 
ing operation  great  care  should  be  taken  that  pieces  of  solder 
do  not  fall  or  run  down  back  of  the  commutator  to  later  pro- 
duce a  short-circuit  and  cause  the  armature  to  be  returned 
for  further  repairs.  To  prevent  damage  to  windings,  acid 
fluxes  should  never  be  used  in  soldering  a  commutator.  A 
solution  of  rosin  in  alcohol  is  recommended  instead.  A  tin 
and  lead  solder  is  considered  best  in  soldering  leads  to  the 
commutator  but  a  pure  tin  solder  is  used  in  making  all  other 


OPERATIONS  BEFORE  AND  AFTER  WINDING         145 

joints  on  the  coils  as  the  insulation  is  less  liable  to  be  damaged 
with  this  solder  on  account  of  its  lower  melting  point.  When 
tin  is  used  the  best  results  are  obtained  by  working  upon 
the  side  of  the  armature,  so  that  the  joint  is  level.  After 
soldering,  the  armature  should  be  mounted  in  a  lathe  and  the 
rough  solder  on  the  necks  of  the  commutator  turned  down, 
the  commutator  polished  and  the  wiper  rings  turned  to  give 
the  exact  distance  between  bearings.  In  some  armatures 
wedges  are  inserted  in  the  slots  above  the  coils.  These  extend 
above  the  surface  of  the  banding  grooves  and  should  be  turned 
down  while  in  the  lathe  if  they  are  used,  so  the  banding  grooves 
will  present  a  smooth  bed  for  the  band  wires. 

Hoods  for  Armatures. — In  order  to  protect  armature  end 
connections  of  railway  motors  and  of  mill  motors  that  must 
be  used  in  places  where  dirt  and  dust  is  liable  to  accumulate 
on  the  armature,  a  heavy  hood  is  often  put  over  the  ends  of 
the  coils.  For  the  commutator  end  a  hood  of  woven  asbestos 
is  suitable.  This  hood  is  usually  sewed  in  a  conical  shape  and 
impregnated  with  a  moisture  and  oil-repelling  compound 
and  fastened  in  place  while  wet.  The  small  end  should  be 
drawn  up  over  the  commutator  and  turned  inside  out  and 
firmly  tied  over  the  coil  leads  and  commutator  necks  with 
h?avy  twine.  The  body  of  the  hood  should  then  be  turned 
back  over  the  armature.  If  the  commutator  necks  are  lower 
than  the  level  of  the  core,  another  layer  of  twine  should  be 
wound  over  the  hood  near  the  commutator  and  a  band  of 
canvas  sewed  over  the  whole.  The  hood  should  then  be 
stretched  tightly  back  over  the  armature  and  tied  with  twine. 

Around  the  rear  end  of  the  armature  a  band  of  canvas  should 
be  wrapped  so  that  the  greater  part  of  the  strip  extends  out 
over  the  shaft  only  enough  being  wound  over  the  armature  to 
permit  a  secure  fastening.  This  should  be  bound  in  place 
with  a  band  of  twine  wound  tightly  in  the  groove  between  the 
coil  ends  and  the  end  bell.  The  canvas  should  then  be  turned 
back  over  the  armature  and  bound  smoothly  in  place. 

In  case  the  armature  is  to  be  banded  with  steel  wire,  two 
strips  of  cotton  tape  separated  by  a  band  of  varnished  paper 
should  be  wound  over  the  hood  and  end  connections  near  the 

core  of  the  armature,  as  a  base  for  the  banding  wires. 
10 


146         ARMATURE  WINDING  AND  MOTOR  REPAIR 

Banding  Armatures. — In  repair  shops  where  banding 
machines  are  not  available,  the  banding  of  armatures  is  done 
in  a  more  or  less  indifferent  way  with  the  result  that  the 
banding  may  only  perform  a  part  of  its  function,  namely, 
to  prevent  the  coils  from  being  thrown  off  the  armature  core. 
Unless  the  banding  is  placed  on  the  armature  with  great  care, 
there  may  be  sufficient  movement  of  the  coils  in  the  slots  to 
wear  the  insulation  and  cause  grounds.  Such  movement  is 
also  liable  to  cause  breaks  in  the  copper  leads  where  they  are 
soldered  rigidly  into  the  commutator.  The  details  for  banding 
an  armature  given  in  what  follows,  are  taken  from  an  article 
in  the  Electric  Journal  and  represent  good  shop  practice  that 
can  be  followed  in  both  large  and  small  repair  shops. 

Shrinking  Coil  Insulation. — Since  the  coil  insulation  shrinks 
upon  being  heated,  it  is  necessary  to  shrink  it  as  much  as 
possible  before  the  final  banding  wire  is  applied.  This  is 
done  by  heating  the  whole  armature  to  about  75°C.  (167°F.), 
when  the  insulation  becomes  pliable  and  can  be  pressed  down 
into  permanent  shape. 

Temporary  Bands. — The  hot  armature  should  then  be 
mounted  in  a  lathe  and  a  protecting  strip  of  cloth  placed  over 
the  end  windings.  Wind  a  temporary  banding  wire  over  the 
coils  with  enough  tension  to  draw  them  down  into  place,  and 
fasten  the  ends  by  soldering  tin  clips  over  the  wire.  The 
armature  should  then  be  allowed  to  cool.  After  the  tem- 
porary wires  are  removed  the  armature  is  ready  for  the 
permanent  banding. 

Banding  Machine. — When  a  banding  machine  is  used,  the 
tension  in  the  wire  is  regulated  by  passing  it  over  a  train  of 
friction  pulleys,  mounted  on  the  carriage.  The  friction  of  the 
pulleys  can  be  adjusted  to  any  desired  value  by  the  regulating 
screws.  In  the  absence  of  such  a  device,  fair  results  can  be 
obtained  by  passing  the  wire  two  or  three  times  around  a 
round  wooden  banding  stick  approximately  two  inches  in 
diameter  and  adjusting  the  tension  by  hand. 

Core  Bands. — When  core  bands  are  used,  the  grooves  should 
be  fitted  with  thin  strips  of  tin,  which  protect  the  coils  from 
the  cutting  action  of  the  bands.  In  starting  the  permanent 
banding,  wind  a  few  turns  at  one  end  fco  secure  the  necessary 


OPERATIONS  BEFORE  AND  AFTER  WINDING        147 

Then  wind  all  the  banding  groups  continuously,  to 
eliminate  the  necessity  of  fastening  the  ends  of  each  group  as 
they  are  wound.  The  bands  should  be  held  together  and  the 
ends  fastened  by  means  of  narrow  tin  strips  (about  0.012  to 
0.02  inch  thick  and  0.25  inch  wide)  placed  under  the  wires 
and  bent  back  over  the  top  and  held  by  pure  tin  solder.  These 
strips  should  be  inserted  while  the  wire  is  being  fed  on  and 
located  about  every  three  inches  around  the  armature,  with 
closer  spacing  at  the  beginning  and  end  of  each  band.  For 
the  core  bands,  these  strips  should  be  placed  in  the  slots  and, 
being  wider  than  the  groove,  they  prevent  any  tendency  for 
the  bands  to  slide  around  the  armature.  The  ends  of  the 
groups  should  then  be  cut  and  secured  by  being  bent  back 
outside  one  clip  and  inside  the  next  one.  Pure  tin  solder 
should  be  applied  to  the  whole  surface  of  the  bands  to  form 
a  solid  web.  ; 

End  Bands. — The  end  windings  should  be  secured  by 
groups  of  wire  wound  on  insulating  hoods  to  protect  the  coils. 
On  the  commutator  end,  strips  of  thin  mica  with  overlapping 
ends  are  usually  placed  on  the  commutator  neck  and  held  in 
place  with  a  few  turns  of  twine.  If  a  hood  or  head  of  canvas 
is  to  be  used  it  should  be  wrapped  around  the  neck,  extending 
about  an  inch  from  the  edge  and  turned  inside  out.  After 
this  end  is  secured  with  twine,  the  free  end  of  the  hood  should 
be  pulled  back  over  the  windings,  bringing  the  outside  of  the 
hood  at  the  surface  and  making  a  neat  folded-under  edge. 
This  hood  can  be  held  temporarily  with  twine  until  the  wire 
is  applied.  The  other  end  of  the  armature  should  be  similarly 
covered  with  a  hood  and  banded. 

Tension  to  be  Applied  to  Band  Wire. — The  proper  tension 
for  banding  wire  when  being  applied  varies  with  the  size  of 
wire  and  the  construction  of  the  end  windings.  When 
the  end  coils  have  no  rigid  support  and  extend  out  a  con- 
siderable distance  from  the  core,  the  tension  should  be  gradu- 
ally reduced,  as  shown  in  the  accompanying  table. 

Wire. — The  best  material  is  a  high-grade  steel  piano  wire, 
having  a  final  breaking  strength  of  200,000  Ib.  per  sq.  in.  For 
temporary  bands  a  cheaper  grade  can  be  used.  The  band 
wire  should  be  tinned. 


148         ARMATURE  WINDING  AND  MOTOR  REPAIR 
POUNDS  TENSION  FOR  BANDING  WIRES 


End  bands 

Diameter  of  wire, 
in. 

Core  bands 

At  core 

At  end  of  wdg. 

0.045 

200 

175 

160 

0  .  0641 

300 

250 

225 

0.0803 

400 

300 

260 

Solder. — Pure  tin  should  be  used,  as  this  gives  a  band  that 
will  hold  together  for  a  longer  time  than  half-and-half  solder. 

Flux. — About  1.5  lb.  of  powdered  rosin,  dissolved  in  one 
quart  of  denatured  or  wood  alcohol  makes  a  good  flux. 

Tin  Clips  and  Strips  should  be  of  commercial  sheet  tin 
about  0.012  to  0.02  inch  thick. 

Precautions. — Use  a  band  wire  that  is  strong  enough  to 
prevent  movement  due  to  high  speed  and  vibration. 

Secure  the  ends  of  all  band  wires  under  the  clips. 

See  that  the  bands  are  below  the  surface  of  the  core  to 
keep  them  from  rubbing  on  the  poles. 

Before  applying  core  bands,  see  that  the  tops  of  the  coils 
are  about  ^2  inch  above  the  band  groove,  so  that  the  bands 
pull  the  coils  down  even  with  the  core.  If  the  coils  are  too 
high  and  the  bands  do  not  rest  on  the  cores,  loose  bands  will 
result  when  the  insulation  dries  out. 

Wind  all  core  bands  in  one  operation. 

In  soldering,  use  a  4-lb.  clean  iron,  well  tinned. 

Bands  to  be  effective  should  be  kept  tight.  If  they  are 
allowed  to  become  loose,  grounded  armatures  and  broken  leads 
may  result.  It  is  considered  good  practice  to  reband  new  or 
newly  rewound  railway  or  hoist  armatures  after  about  12  to 
18  months  as  a  safety-first  measure;  and  to  make  renewals  on 
old  ones  whenever  the  bands  start  to  loosen.  The  service 
conditions — temperature  and  speed — largely  determine  the 
length  of  time  that  bands  will  hold  tight.  One  large  operating 
company  rebands  all  armatures  every  two  years. 

Seasoning  and  Grinding  a  Commutator. — A  new  or  reas- 
sembled commutator  becomes  "seasoned"  that  is  the 
insulation  baked  out  and  all  parts  in  their  final  set  position, 


OPERATIONS  BEFORE  AND  AFTER  WINDING        149 

only  after  being  in  operation  for  a  time  with  the  necessary 
tightening  and  grinding.  This  is  particularly  true  of  a  large 
commutator.  Need  of  attention  to  a  commutator  will  be 
indicated  by  roughness,  high  or  low  bars,  flat  sections  result- 
ing in  poor  commutation.  If  the  commutator  is  in  very  bad 
condition  it  may  be  necessary  to  turn  it  down,  but  for  ordinary 
cases  a  grinding:  tool  is  preferable  and  recommended  such  as 
shown  in  Fig.  115  (Instruction  Book,  Westinghouse  Electric  & 
Mfg.  Co.). 

Commutators  should  always  be  ground  at  from  100  to  120 
per  cent,  normal  speed.  Turning  requires  a  much  lower 
speed;  it  should  not  be  higher  than  150  feet  per  minute. 


FIG.   115. — Grinding  device  for  truing  commutators  when  they  do  not  require 

turning  down. 

Before  grinding  a  commutator,  the  machine  should  have 
been  in  service  a  sufficient  length  of  time  to  bring  the  tem- 
peratures up  to  a  constant  value.  Before  grinding,  the 
brushes  should  be  lifted  off  the  commutator  as  the  copper  and 
stone  dust  will  rapidly  wear  them  off.  The  dust  will  also 
become  imbedded  in  the  brush  contact  surface  and  later 
damage  the  commutator  or  cause  poor  commutation.  The 
armature  winding  should  also  be  thoroughly  protected  during 
this  operation  to  prevent  an  accumulation  of  dirt  and  metal 
chips  which  may  result  in  an  insulation  failure  when  the 
machine  is  again  put  in  service.  This  protection  can  usually 
be  obtained  by  using  a  circular  shield  of  fullerboard,  or  similar 
material,  around  the  commutator  at  the  end  next  to  the 
armature.  This  shield  can  be  easily  supported  from  the 


150         ARMATURE  WINDING  AND  MOTOR  REPAIR 

brush-holder  arms  and  should  extend  from  the  commutator 
surface  to  an  inch  or  two  above  the  surface  of  the  armature. 
It  may  also  be  desirable  to  put  a  temporary  canvas  hood 
over  the  armature  winding.  This  protection  can  be  best 
provided  by  carrying  the  copper  dust  away  by  means  of  a 
vacuum  system.  Even  when  this  is  done  the  armature  should 
be  protected  as  described.  After  grinding,  the  complete 
machine  should  be  thoroughly  cleaned  by  the  methods  already 
described.  It  may  be  nec.essary  to  repeat  the  heating,  tight- 
ening and  grinding  one  or  more  times  before  the  commutator 
is  in  first-class  condition.  Emery  cloth  or  paper  should  never 
be  used  for  this  purpose  on  account  of  the  continued  abrasive 
action  of  the  emery  which  becomes  embedded  in  the  copper 
bars  and  brushes.  Even  when  sandpaper  is  used  the  brushes 
should  be  raised  and  the  commutator  wiped  clean  with  a  piece 
of  canvas  lubricated  with  a  very  small  quantity  of  vaseline  or 
oil.  Cotton  waste  should  never  be  used  and  an  excess  of 
lubricant  should  be  avoided. 

The  grinding  device  shown  in  Fig.  115  can  be  mounted  in  one 
of  the  brush-holder  arms  or  brackets  of  a  large  machine. 
The  grinding  stones  should  be  adjusted  against  the  rotating 
commutator  until  a  clean  cutting  effect  is  secured,  but  should 
be  carefully  shaped  to  the  commutator  surface  before  being 
placed  in  the  grinding  device.  The  stones  can  be  moved 
across  the  surface  of  the  commutator  while  it  is  running,  by 
means  of  the  handle  shown  at  the  right  in  the  illustration. 

Undercutting  Mica  of  Commutator. — Several  devices  and 
hand  tools  are  used  for  this  operation.  Details  of  their  use 
and  descriptions  of  the  devices  are  given  in  Chapter  XII, 
page  320. 

Balancing  an  Armature. — After  an  armature  has  been  wound, 
banded  and  its  commutator  trued  it  must  be  balanced.  In 
the  case  of  small  medium  speed  armatures  this  operation  can 
be  successfully  done  on  a  pair  of  steel  knife  edges  mounted 
parallel  to  each  other  and  perfectly  level.  The  ends  of  the 
armature  shaft  are  placed  on  these  knife  edges  so  that  the 
armature  is  free  to  roll.  It  is  then  given  a  slight  roll  with 
the  hand  and  when  it  comes  to  rest,  the  bottom  marked  with  a 
piece  of  chalk.  This  rolling  and  marking  should  be  repeated 


OPERATIONS  BEFORE  AND  AFTER  WINDING    151 


several  times.  If  the  marks  fall  well  distributed  around  the 
armature  core  the  armature  is  in  sufficient  balance  for  service 
and  can  be  placed  in  the  motor.  In  case  the  marks  fall  close 
together  or  all  on  one  side  of  the  armature,  it  must  be  balanced 
by  adding  weight  to  the  opposite  side  or  removing  weight 
from  the  heavy  side.  This  is  done  in  different  ways  depend- 
ing upon  how  much  the  armature  is  out  of  balance.  Before 
the  weight  is  permanently  fixed  it  must  be  determined  exactly. 
This  can  be  done  by  the  use  of  slugs  of  lead  properly  attached 
and  shaved  down  with  a  knife  until  the  proper  balance  is 
secured.  Frequently  a  nut  or  bolt  head  can  be  filed  on  the 
heavy  side  when  the  amount  to  be  removed  is  small.  Other- 
wise some  method  of  attaching  a  piece  of  metal  or  solder  must 
be  devised  equal  in  weight  to  the  lead  slug  determined  by 
the  test. 

For  high-speed  armatures  and  those  machines  where  a 
perfect  balance  must  be  secured,  the  armature  should  be 
tested  in  a  special  balancing  machine. 

Painting  the  Winding. — The  kinds  of  insulating  varnish 
and  impregnating  compounds  that  should  be  used  on  an  arma- 
ture winding  are  given  on  page  176  of  Chapter  VII.  Only 
good  grades  of  insulation  paints  should  be  used  on  windings. 
When  the  winding  is  subjected  to  acid  fumes,  or  the  machine 
located  in  damp  places  the  manu- 
facturer should  be  consulted  on  the 
treatment  that  should  be  given  the 
winding. 

Relining  Split  Bearings. — The 
following  suggestions  (Lnstruction 
Book,  Westinghouse  Electric  & 
Mfg.  Co.)  can  be  followed  in  re- 
newing split  bearings  of  the  oil  ring 
type.  Melt  the  old  bearing  out 
of  its  shell  and  prepare  an  iron 
mandrel  such  as  shown  in  Fig. 
116  having  the  same  diameter  /  as  the  shaft  and  dimensions 
c,  d  and  e  taken  from  the  bearing  to  be  renewed.  Iron  pieces 
BB  and  C  should  be  attached  by  screws  to  the  mandrel  to 
form  the  oil  ring  slots  and  the  horizontal  inspection  opening 


FIQ.  116.— Mandrel  for 
use  when  relining  split  bear- 
ings. 


152         ARMATURE  WINDING  AND  MOTOR  REPAIR 

in  the  top  half  of  the  bearing.  The  pieces  BB  should  be 
tapered  so  that  they  will  withdraw  easily  from  the  cast  metal. 
A  shpulder  D,  so  placed  as  to  fit  against  the  end  of  the  bear- 
ing shell  serves  as  a  guide. 

For  the  Lower  Half. — Warm  the  madrel  and  bearing  shell 
and  while  both  are  still  warm  so  as  not  to  cool  the  metal  too 
rapidly  when  pouring,  place  the  mandrel  in  the  lower  half  of 
the  shell  with  the  shoulder  D  tight  against  the  end  of  the  shell 
and  the  straight  bottom  portions  of  the  pieces  BB  resting  on 
the  split  plane  of  the  bearing.  Close  the  joints  x  with  putty 
and  fill  the  openings  between  the  shell  and  the  housing  lead- 
ing to  the  oil  well  with  wet  waste.  Pour  the  molten  metal, 
heated  just  enough  to  flow  readily,  into  the  space  between  the 
shell  and  the  mandrel  until  the  metal  is  flush  with  the  split 
surface  of  the  housing.  The  metal  will  harden  very  quickly 
and  the  mandrel  can  then  be  removed. 

For  the  Upper  Half. — Fill  the  openings  in  the  upper  half 
shell  with  putty  and  lay  the  mandrel  in  the  shell  with  the 
split  side  up.  Block  the  openings  leading  to  the  oil  well  with 
waste,  and  pour  as  described  for  the  lower  half.  Remove  the 
mandrel,  smooth  all  rough  edges  in  the  bearing  by  chipping 
or  filing,  and  chip  the  oil  grooves  in  the  lining  of  the  upper 
half.  The  bearing  surface  of  both  halves  should  be  eased  off 
by  scraping  and  the  edges  along  the  split  surface  should  be 
filed  flush  with  the  shell. 


CHAPTER  VII 

INSULATING  COILS  AND   SLOTS  FOR  DIRECT -CUR- 
RENT AND  ALTERNATING -CURRENT  WINDINGS 

On  account  of  the  fact  that  the  voltage  between  commutator 
segments  in  a  1 10- volt  direct-current  machine  is  not  often  more 
than  six  volts,  in  a  600- volt  machine  not  over  18  volts  and  in 
a  1200- volt  machine  a  maximum  of  about  25  volts,  the  voltage 
between  the  conductors  of  an  armature  coil  is  relatively  low. 
These  conductors  are  usually  provided  with  either  a  single, 
double  or  triple  cotton  covering,  the  triple  covering  being  used 
when  the  voltage  between  the  conductors  is  near  the  upper 
limit  of  25  volts.  Where  the  winding  space  is  small  a  silk 
covering  is  sometimes  used  in  small  machines.  After  the  coils 
have  been  formed  and  bound  together  to  hold  the  strands  in 
place,  the  entire  coil  receives  a  special  insulation  as  a  protection 
against  breakdown  between  the  copper  of  the  coil  and  the  iron 
of  the  slot  in  which  it  is  laid.  The  insulation  for  small  low 
voltage  machines  may  consist  of  a  wrapper  of  treated  cloth 
or  mica  held  in  place  by  a  layer  of  cotton  tape  wound  so  that 
it  does  not  overlap.  The  end  connections  of  the  coil  are  then 
protected  by  an  overlapping  layer  of  cotton  tape  with  cotton 
sleeves  used  on  the  leads  as  further  protection.  The  entire 
coil  is  then  dipped  in  an  insulating  varnish  and  baked. 

Insulation  for  Armature  Coils  and  Slots. — In  addition  to  the 
insulation  provided  on  the  coils,  it  has  been  found  necessary 
to  pay  particular  attention  to  the  insulation  of  the  winding  as  a 
whole  from  the  armature  core.  The  insulation  used  performs 
two  functions,  namely,  to  provide  mechanical  protection  and 
to  serve  as  electrical  insulation.  The  following  classification 
can  be  made  of  the  materials  that  are  much  used  as  slot  and 
coil  insulation: 

For  Mechanical  Protection. — Pressboard,  presspahn,  vul- 
canized fiber,  hard  fiber,  fish  paper,  rope  paper,  Japanese 
and  Manila  paper. 

153 


154         ARMATURE  WINDING  AND  MOTOR  REPAIR 

For  High  Temperatures  and  Electrical  Insulation. — Mica, 
micanite,  mica  paper  and  mica  cloth. 

For  Electrical  Insulation  Only. — Cotton  tape  and  oiled  or 
treated  cloth  which  includes  cotton  or  linen  muslin,  varnished 
cambric,  varnished  muslin,  and  empire  cloth. 


FIG.  117. — Type  of  wire  wound  coils  used  in  stator  of  Fairbanks-Morse 
alternators  (Fig.  155).  These  coils  are  thoroughly  insulated  and  require 
only  a  non-abrasive  material  in  the  slot  to  give  mechanical  protection  to  the 
coils. 

For   Mechanical   Protection   and   Electrical   Insulation. — 

Pressboard  or  fullerboard  is  a  material  resembling  cardboard 
and  made  from  cotton  rags  and  paper  clippings.  It  varies 
from  seven  to  125  mils  in  thickness  and  when  properly  treated 
and  varnished  has  a  dielectric  strength  of  about  500  volts  per 
mil  in  thickness  up  to  25  mils  and  then  reducing  to  about  200 
volts  per  mil  in  the  thicker  sheets. 

Presspahn  is  the  name  given  to  the  pressboard  which  is  made 
in  Germany. 

Vulcanized  fiber  also  known  as  hard  fiber  is  a  dense  hard 
material  with  a  dielectic  strength  of  about  200  volts  per  mil 
at  thicknesses  of  from  50  to  150  mils.  It  is  used  wherever 
an  insulating  material  of  exceptional  mechanical  strength  is 
needed  such  as  wedges  in  armature  slots  and  coil  braces. 

Horn  fiber  is  a  material  made  in  different  colors  that  has 
a  high  tensile  strength  and  also  a  good  dielectric  strength 
(250  volts  per  mil  when  10  mils  thick)  which  can  be  increased 
by  impregnation  with  oil  or  varnish. 

Fish  paper  is  made  from  rag  stock  and  through  a  treating 
process  becomes  a  hard  fiber-like  paper  which  is  very  strong. 
This  material  is  not  affected  by  heat  and  on  account  of  this 


INSULATING  COILS  AND  SLOTS 


155 


and  its  mechanical  strength  is  much  used  as  cell  lining  in 
armature  slots. 

Manila  paper  is  made  from  linen  or  Manila  fiber  producing  a 
tough  strong  paper  which  when  dry  has  a  dielectric  strength 
varying  from  100  to  230  volts  per  mil  in  thicknesses  of  from 
1.8  to  28  mils. 

For  High  Temperatures  and  Electrical  Insulation. — Mica 
is  one  of  the  very  few  materials  which  maintains  a  high  dielec- 
tric strength  at  high  temperatures.  It  is  not,  however,  me- 
chanically strong.  Flexible  sheets  of  mica  are  made  up  by 
sticking  thin  splittings  of  mica  on  one  side  of  a  sheet  of  paper 
or  cloth  with  a  suitable  varnish  in  such  a  way  that  the  joints 


FIG.  118. — Different  types  of  coils  used  in  rewinding  motors. 

(1),  (2),  and  (3)  are  wire  wound  direct-current  coils.  Two  sections  are  shown  in  (l) 
before  assembling  the  complete  coil  in  (2).  The  coil  in  (3)  is  ready  to  be  spread  into  a 
diamond  shape  in  a  pulling  machine.  (4)  is  a  so-called  mush  or  basket  coil  for  partially 
closed  slots.  The  coil  as  shown  is  ready  for  use  in  A.  C.  motors  from  about  H  to  15  hp. 
(5)  is  a  completed  strap  coil  ready  for  final  treatment  or  dipping.  (6)  is  a  shuttle 
wound  coil,  three  wires  wide  by  eight  wires  deep  in  series  for  A.  C.  motors  with  open 
slots.  This  coil  is  spread  into  shape  in  a  pulling  machine.  It  is  insulated  with 
varnished  cambric  at  the  end  where  wires  cross  in  winding.  The  coil  at  (7)  is  the  same 
as  (6)  after  being  spread  and  insulated  with  tape. 

are  staggered.  Such  built-up  sheets  are  known  under  different 
names,  such  as  "  Japanese  Paper  and  Mica,"  "Fish  Paper  and 
Mica"  and  " Treated  Cloth  and  Mica."  These  are  also 
referred  to  as  Mica  paper  and  Mica  cloth. 

Micanite  is  a  form  of  reconstructed  mica  made  both  in  plate 
and  flexible  forms.  The  latter  is  used  for  armature  slots  and 
the  former  for  commutator  segment  insulation.  Its  dielectric 
strength  is  very  high  ranging  in  the  flexible  form  from  about 
600  volts  per  mil  for  five  mil  thickness  to  500  volts  per  mil  in 


156         ARMATURE  WINDING  AND  MOTOR  REPAIR 

125-mil  thicknesses.  Micanite  paper  and  Micanite  cloth  is 
also  made  with  Japanese  paper  and  with  muslin. 

For  Electrical  Insulation  Only. — Cotton  is  used  in  the  form 
of  tape  or  cloth.  When  used  primarily  for  its  dielectric 
strength,  it  is  treated  with  an  insulating  compound.  After 
such  treatment  cotton  cloth  will  withstand  about  1000  volts 
per  mil  thickness.  Because  this  insulation  is  flexible  and 
tough  it  is 'much  used  as  an  insulation  in  the  insulating  of  coils 
and  other  parts  of  electrical  machinery.  Since  ifc  is  quite 
susceptible  to  damage  and  abrasion  it  is  mostly  used  with  a 
protective  covering  such  as  untreated  cotton  tape  or  friction 
tape  or  a  tough  paper  such  as  fish  paper.  Different  weights 
and  thicknesses  of  cloth  are  used  for  insulating  purposes  such 
as  cambric  five  mils  thick; muslin  eight  mils  thick; heavy  cotton 
11  mils  thick;  drilling  about  17  mils  thick;  and  duck  about  30 
mils  thick.  When  duck  is  treated  with  linseed  oil  or  varnish 
it  is  frequently  used  as  a  protective  covering  over  coil  supports, 
between  coil  ends  and  over  the  ends  of  armatures.  Cotton 
insulating  material  when  treated  with  an  insulating  compound, 
varnish  or  linseed  oil,  is  known  under  several  names  such  as  var- 
nished cambric,  varnished  muslin,  Empire  cloth,  Kabak  cloth,  etc. 

Any  of  the  cotton  materials  can  be  cut  into  narrow  widths 
for  use  as  tape  when  insulation  is  more  important  than  me- 
chanical strength.  Tape  cut  on  the  bias  is  used  to  tape-up 
coils  of  irregular  shapes. 

Descriptions  and  Uses  of  Insulating  Materials. — In  the 
following  paragraphs  the  composition  of  the  insulating  ma- 
terials already  mentioned  together  with  many  others  that  are 
available  are  given  with  their  various  uses.  This  information 
is  taken  from  a  comprehensive  classification  of  treated  cloths, 
pressboards,  fibers  and  papers  by  Hugh  E.  Weightman, 
Chief  Engineer,  Engineering  Service  Company,  Chicago,  111. 
(Electrical  Record,  July,  1919). 

Treated  Cloths. — The  different  kinds  of  treated  cloths 
which  are  available  are  as  follows: 

Black  varnished  cambric  Yellow  varnished  cambric 

Japanned  muslin  Oiled  muslin 

Japanned  duck  Yellow  oiled  canvas 

Varnished  silk  Yellow  oiled  cotton  drill 


INSULATING  COILS  AND  SLOTS  157 

Uses  and  Properties  of  Treated  Cloths. — Each  of  the  materials 
in  the  foregoing  list  has  somewhat  different  insulating  pro- 
perties. Black  varnish  cambric  is  a  varnish  coated  cloth 
used  in  the  form  of  straight-cut  tape  for  wrapping  wire  and 
cables  and  as  bias  cut  tape  for  armature  coils.  It  is  usually 
supplied  in  0.010-in.  and  0.012-in.  thicknesses  in  rolls  about 
36  in.  wide.  It  is  also  supplied  in  ready  cut  tapes.  The 
material  is  obtainable  in  two  or  more  grades,  the  cheaper 
grades  being  used  for  phase  insulation. 

Japan  muslin  is  unbleached  muslin  cloth  treated  with  black 
japan  and  baked  to  produce  a  waterproof  material  of  good 
insulating  properties.  It  is  used  for  wrapping  where  a  coarse 
cloth  is  permissible  and  is  usually  supplied  by  the  manufacturer 
in  0.017-in.  thick  by  30-in.  wide  rolls. 

Japan  duck  is  a  high  grade  8-oz.  duck  approximately 
0.025  in.  thick  and  of  close  weave.  It  is  treated  with  japan 
and  oven  cured.  It  is  chiefly  used  under  the  binding  bands 
of  railway  motors  as  a  protecting  and  moisture  excluding 
fabric.  This  material  is  usually  supplied  in  rolls  36  in. 
wide. 

Varnished  silk  is  made  of  Japanese  silk  treated  with  a 
high  grade  insulating  varnish  and  oven  cured.  This  makes 
a  thin,  tough  insulating  material  of  high  dielectric  strength 
which  is  used  where  light  weight  and  a  minimum  of  thickness 
is  required.  Varnished  silk  is  also  employed  on  meter  coils 
and  as  insulation  in  airplane  apparatus.  The  material  in 
addition  to  being  light  does  not  become  brittle  in  extreme  cold 
nor  gummy  in  heat.  It  is  quite  expensive  and  for  that 
reason  is  not  generally  applicable.  The  usual  thicknesses 
are  0.003  in.  and  0.005  in.  Sheets  are  27  in.  wide. 

Yellow  varnished  cambric  is  a  strong,  closely  woven  cotton 
cloth  having  an  especially  soft  finish  and  treated  with  high 
grade  varnish.  The  varnish  is  baked  in  place,  producing 
a  material  having  a  very  high  dielectric  strength  and  a  hard 
smooth  surface.  Its  insulation  resistance  is  usually  not  as 
high  as  black  varnished  cambric,  especially  at  high  tempera- 
tures. It  is  more  easily  handled  than  the  black  varnished 
cambric.  It  is  usually  supplied  in  rolls  and  sheets  36  in.  wide 
and  in  tapes  straight-cut  or  bias-cut,  0.010  in.  and  0.012  in, 


158         ARMATURE  WINDING  AND  MOTOR  REPAIR 

thick.  This  material  is  used  for  much  the  same  purposes  as 
black  varnished  cambric. 

Oiled  muslin  is  a  linen  finished  cloth  coated  with  oil  and 
oven  cured  to  set  the  film  to  a  hard  smooth  surface.  The 
product  is  a  flexible  cloth  having  high  insulating  properties 
and  good  resistance  to  deterioration  through  vibration  or 
aging.  It  is  used  for  a  large  variety  of  purposes,  especially 
for  wrapping  armature  coils  and,  in  tape  form,  for  taping  coils 
and  leads.  It  is  supplied  0.007  in.  thick  in  36-in.  rolls  or  in 
standard  width  tapes. 

Yellow  oiled  canvas  is  a  high  grade  duck  treated  with 
oil  to  produce  a  flexible  waterproof  material.  It  is  used 
for  pads  under  railway  motor  field  coils  and  in  similar  places. 
This  material  is  commonly  obtained  in  one  thickness  of  0.045 
in.  in  rolls  36  in.  wide.  It  is  also  used  for  tarpaulins  for  genera- 
tors, switches  and  such  apparatus.  Yellow  oiled  cotton  drill 
is  a  light  unbleached  cotton  drill  treated  with  oil  and  oven 
cured  to  set  the  film  to  a  firm  surface.  It  is  made  0.017  in. 
thick  and  in  rolls  30  in.  wide.  It  is  often  used  for  coil  separa- 
tors in  transformers  and  regulators. 

Pressboards,  Fibres  and  Papers. — Fibers,  leatheroids  and 
such  materials,  are  especially  selected  for  their  high  insulating 
properties,  and  they  are  treated  to  render  them  more  pliable 
and  easily  worked.  The  treatment  does  not  appreciably 
affect  the  thickness  except  in  the  case  of  the  shellacked 
materials.  This  treatment  adds  approximately  0.005  in. 
in  thickness  to  the  size  of  each  sheet.  When  ordering  treated 
material  from  the  manufacturer  the  thickness  always  refers 
to  the  untreated  material.  The  following  kinds  of  this  class 
of  treated  materials  are  available: 

Pressboard  Varnished  rawhide  fiber 

Japanned  pressboard  Leatheroid 

Oiled  pressboard  Express  parchment  paper 

Shellacked  pressboard  Shellacked  express  paper 

Varnished  pressboard  Varnished  express  paper 

Horn  fiber  Red  rope  paper 

Japanned  horn  fiber  Oiled  red  rope  paper 

Oiled  horn  fiber  Shellacked  red  rope  paper 

Shellacked  horn  fiber  Varnished  red  rope  paper 

Varnished  horn  fiber  Shellacked  bond  paper 


INSULATING  COILS  AND  SLOTS  159 

Rawhide  fiber  Asbestos  paper 

Japanned  rawhide  fiber  Oiled  asbestos  paper 

Oiled  rawhide  fiber  Varnished  asbestos  paper 
Shellacked  rawhide  fiber 

Pressboards. — Pressboard  is  a  specially  prepared  paper  which 
is  dense,  flexible  and  easily  worked.  It  readily  absorbs  oils 
and  varnishes  which  render  it  less  hygroscopic  than  in  its 
untreated  state.  It  is  used  very  extensively  as  layer  insulation 
and  spacers  in  various  types  of  generator,  motor,  transformer 
and  regulator  coils.  Collars  and  shields  on  high-voltage 
transformers  are  also  made  of  this  form  of  insulation.  One 
manufacturer  carries  this  material  in  stock  in  the  following 
sizes : 

Thickness  in  inches  Size  of  sheets  in  inches 

0.009  30  X  40 

0.020  33  X64 
0.030                                           "v      34X40 

0.030  36X84 

0.60  34X40 

0.60  36X84 

^2  24  X  60 

H  40  X  60 

Japanned  pressboard  is  used  for  separators  and  fillers 
in  armature  and  field  coils.  It  is  made  in  the  same  sizes 
listed  in  the  preceding  paragraph.  Oiled  pressboard  is  used 
in  the  same  way  as  japanned  pressboard,  and  it  is  sometimes 
preferred  since  it  is  more  flexible.  It  is  supplied  in  the  same 
sizes  as  given  for  pressboard.  Shellacked  pressboard  finds 
its  principal  application  as  separators  in  bonding  railway 
armature  coils.  The  shellac  upon  the  application  of  heat, 
melts,  forming  a  close  union  between  the  separator  and  the  coil. 
The  principle  thickness  used  is  10  mils  or  0.010  in.  although 
other  sizes  are  available.  This  material  is  often  made  up 
locally  by  manufacturers.  Any  sizes  previously  listed  are 
available. 

Pressboard  varnished  and  oven  cured  is  used  for  fillers  and 
separators.  The  usual  sizes  are  available  as  listed  above. 

Horn  Fibers. — Horn  fiber  is  a  tough  flexible  insulating 
material  of  high  mechanical  and  dielectric  strength.  It  is 
used  for  separators,  slot  channels,  angles  and  joint  insulation. 


160         ARMATURE  WINDING  AND  MOTOR  REPAIR 

and  wherever  a  high  degree  of  flexibility  is  essential.  This 
material  can  be  bought  in  both  sheets  and  rolls.  All  horn 
fibers  are  obtainable  in  the  same  thicknesses  as  listed  for 
pressboard.  Japanning  greatly  increases  the  flexibility  and 
dielectric  strength  of  horn  fiber.  One  special  application  of 
japanned  horn  fiber  is  as  fillers  for  bracket  insulation  and 
around  rocker  arms. 

Treated  with  oil  and  oven  cured  horn  fiber  makes  a  superior 
dielectric  material  for  general  work.  Shellacked  horn  fiber 
finds  its  greatest  application  as  slot  armors,  it  being  customary 
to  form  the  armors  before  shellacking.  Varnished  horn  fiber 
is  used  for  fillers  and  separators  in  preference  to  pressboard 
on  better  machines  and  especially  on  high  speed  units  because 
it  is  more  flexible  and  does  not  disintegrate  as  rapidly  when 
subjected  to  vibration. 

Rawhide  Fibers. — Rawhide  fibre  is  a  harder  pressed  material 
than  horn  fiber  and  consequently  lacks  some  of  the  flexibility 
of  the  latter.  It  is  very  tough  and  can  be  rendered  more 
flexible  by  treatment  with  japan,  oil  shellac  or  varnish. 
Untreated  it  is  used  where  toughness  is  essential  and  where 
great  flexibility  is  not  required.  The  thicknesses  of  the  stock 
sizes  of  the  different  forms  of  rawhide  fiber  usually  found  on  the 
market  are:  0.005  in.,  0.010  in.,  0.015  in.  and  0.020  in.  Sheets 
or  rolls  are  40  in.  to  48  in.  wide.  Sheets  are  usually  72  in. 
to  120  in.  long.  Japanned  rawhide  fiber  has  increased  diel- 
ectric strength  and  is  more  flexible  than  the  untreated  fiber. 
It  is  used  for  the  more  simple  shapes  of  slot  armor  and  for 
fillers  and  separators.  Rawhide  fiber  treated  with  a  high 
grade  oil  and  oven  cured  to  produce  a  firm  surface  is  used  for 
the  same  purposes. 

Shellacked  rawhide  fiber  is  used  for  the  same  purposes  as 
shellacked  horn  fiber,  where  great  flexibility  is  not  required. 
Varnished  rawhide  fiber  is  the  most  flexible  of  the  rawhide 
fibers  and  is  used  for  fillers  and  separators. 

Leatheroids. — In  appearance  leatheroid  is  quite  similar  to 
rawhide  fiber.  It  is  used  for  the  same  general  purposes  as 
the  rawhide  and  horn  fiber.  It  is,  however,  more  resistant 
to  heat,  is  more  dense  and  molds  more  readily  than  either  of 
the  others.  The  usual  thicknesses  available  in  sheets  or  rolls 


INSULATING  COILS  AND  SLOTS  161 

42  in.  wide  are  0.010  in.,  0.015  in.,  0.020  in.,  0.030  in.,  0.060  in., 
and  %  m-  The  sizes  apply  to  all  forms  of  leatheroids.  The 
japanned  leatheroid  is  used  similarly  to  japanned  horn  or 
rawhide  fiber  where  these  materials  are  unable  to  stand  the 
heat.  Oiled  leatheroid  is  used  in  place  of  oiled  horn  or  raw- 
hide fiber  where  a  greater  resistance  to  heat  is  desirable. 

Paper  Insulation. — Express  parchment  paper  is  a  strong 
high-grade  wood  fiber  paper  selected  to  insure  freedom  from 
pin  holes  and  metallic  particles.  It  is  used  for  layer  insulation 
and  in  making  up  pasted  mica  sheets.  The  commercial 
sizes  are  as  follows: 

Rolls  0.003  in.  thick,  30  in.  wide 

Rolls  0 . 005  in.  thick,  32  in.  wide 

Rolls  0. 009  in.  thick,  32  in.  wide 

Sheets  0.005  in.  thick,  30  in.  X  35  in.  wide 

Sheets  0.009  in.  thick,  30  in.  X   35  in.  wide. 

When  it  is  shellacked,  express  parchment  paper  is  used 
for  armature  and  field  coil  bonding.  The  5-  and  9-mil 
sizes  are  most  frequently  used  for  this  purpose.  Varnished 
express  parchment  paper  is  used  chiefly  for  slot  insulation, 
its  best  recommendation  being  its  glossy  surface  to  which  coils 
will  not  readily  adhere  when  putting  them  in  place.  The 
same  commercial  sizes  are  available. 

Red  rope  paper  of  a  good  hemp  rope  stock,  selected  to 
avoid  pin  holes  and  metallic  particles  is  used  in  conjunction 
with  parchment  paper  and  other  insulations  in  building 
up  layer  insulations.  Treated  with  hot  oil  red  rope  paper  is 
excellent  for  troughs,  slot  insulation,  etc.  Where  heavy  bar 
wound  armature  coils  are  used  which  are  subjected  to  steam 
mold  bonding  shellacked  red  rope  paper  is  usually  used.  The 
sizes  listed  for  express  parchment  apply  equally  to  these  red 
rope  papers.  The  5-  and  9-mil  sizes  of  shellacked  red  rope 
paper  are  the  most  used.  Red  rope  paper  varnished  and 
baked  is  used  principally  for  slot  insulation.  As  a  rule  it  is 
made  only  in  5-  and  9-mil  thicknesses  in  sheets  30  in.  by  55  in. 

Shellacked  bond  paper  is  bond  paper  treated  with  a  medium 

bodied  shellac  solution.     It  is  used  for  separators  in  steam 

molds  and  is  usually  obtainable  in  rolls  of  any  desired  width. 

The  thickness  is  0.010  in.  or  10  mils.     Asbestos  paper  is  used 

u 


162         ARMATURE  WINDING  AND  MOTOR  REPAIR 

in  making  layer  insulations  where  heat  resistance  is  needed. 
This  material  must  be  purchased  under  specifications  having  a 
test  or  inspection  clause  in  order  to  guard  against  the  presence 
of  conducting  particles  of  iron  oxide.  It  is  furnished  usually 
in  rolls  36  in.  wide  and  in  the  following  thicknesses:  6  mil, 
15  mil  and  20  mil,  J£2  m->  KG  in.  and  ^  in.  Asbestos  paper 
treated  with  hot  oil  and  oven  cured  is  used  in  railway  motor, 
field  and  armature  coils  and  in  machinery  operating  at  high 
temperatures. 

When  the  asbestos  paper  is  treated  with  a  special  black 
plastic  varnish  and  oven  cured  it  becomes  a  great  moisture 
proof  insulation  and  is  used  in  forms  for  cores  or  spools.  It 
is  used  as  a  moisture-proof,  heat-resisting  insulation  for  motors 
intended  for  naval,  mine  or  similar  services. 

Coil  and  Slot  Insulation  Used  in  One  Large  Repair  Shop. — 
The  following  grades  and  thicknesses  of  coil  and  slot  insulation 
is  carried  in  stock  and  used  by  one  large  repair  shop  where  all 
types,  makes  and  sizes  of  motors  are  rewound. 

Fish  Paper:  In  sheets  4,  7,  10,  15  and  23  mils  thick. 

Fish  Paper  with  Mica  Splittings:  In  sheets  13  mils  thick. 

Fullerboard:  In  sheets  7,  10,  and  15  mils  thick. 

Shellacked  Fullerboard:  In  sheets  7,  10  and  15  mils  thick. 

Treated  Cement  Paper  and  Mica  Splittings:  In  sheets  14  mils 
thick. 

Barren  Paper:  In  sheets  3  and  5  mils  thick. 

Empire  Cloth:  In  sheets  9  mils  thick. 

Cotton  Duck:  In  sheets  12  mils  thick. 

Cotton  Tape:  In  widths  %  and  1}^  in.,  7  mils  thick. 

Treated  Cloth  Tape:  In  width  %  in.,  8  mils  thick. 

Under  the  different  headings  of  Chapter  III  dealing  with  the 
windings  of  particular  machines,  details  are  given  concerning 
the  uses  of  the  different  insulating  materials  described  here. 

Micartafolium. — A  special  insulating  material  has  been 
developed  by  the  Westinghouse  Electric  &  Manufacturing 
Company  which  is  known  as  micarta.  In  the  form  of 
micartafolium  it  can  be  used  as  a  wrapping.  This  insulation 
is  used  on  the  larger  alternating-current  armature  windings, 
particularly  those  of  alternating-current  turbo-generators. 
It  is  also  used  for  direct-current  armature  windings.  For 


INSULATING  COILS  AND  SLOTS  163 

commutators  it  is  used  between  the  bottom  of  the  segments 
and  the  sleeve  on  the  driving  spindle.  It  is  also  used  in 
direct-current  machines  for  flash  guards  between  the  com- 
mutators and  the  armature  winding  of  large  units.  For 
insulating  studs  and  the  capping  of  nuts  and  bolt  heads  it 
has  also  been  found  satisfactory.  This  type  of  insulation 
is  made  in  sheets,  blocks  and  tubes. 

Thickness  of  Insulation  Required  in  Slots. — Prof.  Alfred  Still 
in  his  book  on  Electrical  Design,  under  the  heading  of  slot 
lining  points  out  that  the  insulation  may  be  placed  around 
the  individual  coils  or  in  the  slot  before  the  coils  are  inserted. 
Part  of  the  insulation  may  also  be  placed  around  the  coils 
and  the  remainder  in  the  form  of  slot  lining.  The  essential 
thing  is  to  have  sufficient  thickness  of  insulation  between  the 
cotton-covered  conductors  and  the  sides  of  the  slot.  In  this 
connection  the  following  figures  are  given  by  Prof.  Still  for 
the  thickness  of  slot  lining  required  for  machines  of  different 
voltages.  The  thickness  in  inches  is  for  one  side  only  of  the 
slot,  and  is  as  well  the  thickness  that  should  be  provided  be- 
tween the  upper  and  lower  coil  sides  in  the  slot. 

Oper  i  ting  voltage  Slot  insulation — one  side 

for  direct-current  machines 

AI  B« 

Up  to     250  0.035  inch 

Up  to     500  0 . 045  inch               0 . 035  inch 

Up  to   1000  0.060  inch               0.055  inch 

Up  to   1500  0.075  inch 

For  alternating-current  machines 

500  to    1,500,  same  as  for  direct-current  machines 
Up  to    2,000  0.080  inch  0.090  inch 

Up  to    4,000  0 . 120  inch  0 . 130  inch 

Up  to    8,000  0.190  inch  0.185  inch 

Up  to  12,000  0 . 270  inch  0 . 220  inch 

Insulation  of  Formed  Coils. — The  practice  of  most  motor 
manufacturing  companies  is  to  insulate  coils  for  two  or  three 
ranges  of  voltage.  In  making  changes  in  the  connections  of 

1  Prof.  Alfred  Still — Principles  of  Electrical  Design. 

2  H.  M.  Hobart — Design  of  Polyphase  Generators  and  Motors. 


164         ARMATURE  WINDING  AND  MOTOR  REPAIR 

induction  motors,  therefore,  where  the  changes  involves  an 
increase  in  voltage,  it  is  important  to  know  that  the  changes 
do  not  subject  the  coils  to  a  voltage  outside  the  range  for 
which  they  were  insulated.  This  precaution  does  not,  of 
course,  apply  when  reconnecting  for  a  lower  voltage. 


FIG.  119. — Diamond  shaped  coil  insulated  and  impregnated  with  compound 
ready  for  use  (Westinghouse  Electric  &  Mfg.  Company). 

The  practice  of  one  large  manufacturer  is  to  insulate  coils 
for  three  ranges  of  voltage  as  follows: 

/.  Direct-current  and  Alternating-current  Group  Coils  for  Vol- 
tages up  to  220. 

For  plain  coils — other  than  phase  coils : 

1.  Wound  with  double  cotton-covered  wire. 

2.  Binding  paper  over  slot  side,  0.003  inch  thick. 

3.  1^  turns  treated  cloth  on  slot  side,  0.008  inch  thick. 

4.  Cotton  tape,  one-half  lapped  all  around,  0.004  to  0.007  inch 
thick. 

5.  Entire  coil  dipped  in  insulating  compound  and  baked. 

For  phase  coils  the  same  insulation  is  provided  and  in  addition  a 
layer  of  empire  cloth  and  tape  over  each  end  of  the  coil  up  to  the 
straight  sides.  The  diamon'd  points  are  painted  red  to  dis- 
tinguish these  coils  from  plain  coils.  Phase  coils  are  used  at 
beginning  and  end  of  each  phase  group. 

II.  Direct-current  and  Alternating-current  Group  Coils  for  Vol- 
tages up  to  500. 

For  plain  coils — other  than  phase  coils: 

1.  Wound  with  double  cotton-covered  wire. 

2.  Binding  paper  over  slot  side,  0.003  inch  thick. 

3.  1^  turns  mica  and  fish  paper  on  slot  side,  0.012  inch  thick. 
This  is  made  up  of  fish  paper,  0.004  inch;  three  layers  mica 
splittings,  each  0.001  to  0.003  inch  thick ;  Japanese  paper,  0.001 
inch  thick.     All  shellacked  together. 


INSULATING  COILS  AND  SLOTS  165 

4.  Cotton  tape,  one-half  lapped  all  around,  0.007  inch  thick. 

5.  Entire  coil  dipped  in  insulating  compound  and  baked. 

For  phase  coils  the  insulation  of  ends  is  the  same  as  for  220  volts  of 
the  first  group. 

Ill .  Alternating-current  Group  Coils  for  Voltages  from  500  to 
2300. 

For  plain  coils — other  than  phase  coils : 

These  coils  are  insulated  the  same  as  group  II  except  3^  turns 
instead  of  1^  turns  of  fish  paper  and  mica  are  used.  A  layer 
of  treated  cloth  on  the  ends  of  the  coils  is  also  used. 

For  phase  coils  two  layers  of  treated  cloth  and  a  layer  of  cotton  tape 
are  added. 

Insulation  for  Coils  used  in  240-Volt  and  500-Volt  Direct- 
current  Machines. — The  following  recommendations  are  given 
by  Prof.  Alexander  Gray  (Electrical  Machine  Design,  Chapter 
IV  on  Insulation)  for  the  windings  of  240-  and  500- volt  direct- 
current  machines. 

For  a  240-volt,  double-layer  winding  with  two  turns  per  coil 
and  four  coil  sides  (eight  conductors)  per  slot,  the  conductors 
being  of  strip  copper  wound  on  edge: 

(a)  After  the  copper  has  been  bent  to  shape,  tape  it  all  over  with  one 
layer  of  half-lap  cotton  tape  6  mils  thick.  This  forms  the  insulation 
between  the  adjacent  conductors  in  the  same  slot. 

(6)  Tape  together  the  two  coil  sides  when  they  form  one  group  (as  in 
this  case)  with  one  layer  of  half-lap  cotton  tape  6  mils  thick  all  around 
the  coils.  This  forms  the  end-connection  insulation  and  also  part  of  the 
slot  insulation. 

(c)  Bake  the  coil  in  a  vacuum  tank  at  100°C.  (212°F.)  so  as  to  expel 
all  moisture,  then  dip  it  into  a  tank  of  impregnating  compound  at  120°C. 
(248°F.)  and  leave  it  there  long  enough  to  become  saturated  with  the 
compound. 

(d)  Put  one  turn  of  empire  cloth  10  mils  thick  on  the  slot  part  of  the 
coil  and  lap  the  ends.     This  insulation  should  extend  %  inch  past  the 
ends  of  the  slot. 

(e)  Put  one  turn  of  insulating  paper  10  mils  thick  on  the  slot  part  of 
the  coil  and  lap  it  over  at  the  ends.     This  paper  should  be  long  enough 
to  also  extend  past  the  edge  of  the  slot  %  inch  on  each  end.     It  is  not 
put  on  for  insulating  purposes  but  to  protect  the  other  insulation  which 
is  liable  to  become  damaged  when  the  coils  are  being  placed  in  the  slots. 

(/)  Heat  the  coil  to  100°C.  (212°F.)  and  then  press  the  slot  part  to 
shape  while  hot.  The  heat  softens  the  compound  and  the  pressing  forces 
out  all  excess  of  compound.  The  coil  can  then  be  allowed  to  cool  while 


166 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


INSULATING  COILS  AND  SLOTS  167 

under  pressure  and  will  come  out  of  the  press  with  such  a  shape  and  size 
that  it  will  slip  easily  into  the  slot. 

(gr)  Dip  the  ends  of  the  coil  in  elastic  insulating  varnish. 

For  a  500-volt,  double-layer  winding  having  five  turns  per 
coil,  and  thirty  conductors  per  slot,  with  the  conductors  covered 
with  double-cotton  coverings: 

(a)  Put  one  turn  of  insulating  paper  5  mils  thick  around  the  slot 
part  of  the  two  conductors  that  form  the  individual  coils.  This  paper 
should  extend  %  inch  beyond  the  edge  of  the  slot  at  each  end.  It  forms 
part  of  the  insulation  between  the  individual  coils  in  the  same  slot  and 
also  part  of  the  insulation  from  winding  to  core. 

(6)  Put  one  turn  of  empire  cloth  10  mils  thick  around  the  three  coils 
that  form  one  group  and  lap  it  over  at  the  ends.  Allow  this  insulation 
to  extend  as  before  %  inch  at  each  end  of  the  slot. 

(c)  Put  one  turn  of  paper  5  mils  thick  on  the  slot  part  of  the  coil  and 
lap  it  on  the  top.     As  in  the  other  cases  allow  the  paper  to  extend  over 
the  slot  length  %  inch. 

(d)  Tape  the  ends  of  the  groups  of  three  coils  with  one  layer  of  half- 
lap  cotton  tape  6  mils  thick  and  carry  this  tape  on  to  the  paper  for  ^ 
inch  to  seal  the  coil. 

(e)  Wind  the  machine  with  these  coil  groups,  putting  a  lining  of  paper 
10  mils  thick  in  the  slot  and  then  hold  the  coils  down  with  band  wires. 

(/)  Place  the  armature  in  a  vacuum  tank  and  bake  it  at  100°C.  (212°F.) 
to  expel  moisture,  then  force  impregnating  compound  into  the  tank  at  a 
pressure  of  60  Ib.  per  square  inch  and  maintain  this  pressure  for  several 
hours  until  the  winding  has  been  thoroughly  impregnated. 

(g)  Rotate  the  armature  while  it  is  still  hot  at  a  high  speed  so  as  to 
get  rid  of  the  excess  of  compound  which  will  otherwise  come  out  when  the 
machine  is  carrying  a  heavy  load. 

(h)  Paint  the  end  connections  with  elastic  finishing  varnish,  taking 
care  to  get  into  all  the  corners. 

Coil  Insulation  for  Induction  Motor  Windings. — The  fol- 
lowing recommendations  are  given  by  Prof.  Alexander  Gray 
(Electrical  Machine  Design,  Chapter  XX  on  A.-C.  Insulation) 
for  the  coil  and  slot  insulation  of  induction  motors : 

For  440-volt,  wire-wound  coils  of  double-layer  windings: 

(a)  Double  cotton  covering  on  the  conductors. 

(6)  A  layer  of  empire  cloth  six  mils  thick  between  horizontal  layers  of 
conductors. 

(c)  One  turn  of  paper  10  mils  thick  on  the  slot  part  of  the  coil  to  hold 
the  conductors  in  layers. 

(d)  One  layer  of  half-lapped  empire  cloth  tape  six  mils  thick  all  around 
the  coil. 


168         ARMATURE  WINDING  AND  MOTOR  REPAIR 

(e)  One  turn  of  paper  10  mils  thick  on  the  slot  part  of  the  coil  to  protect 
the  empire  cloth. 

(/)  One  layer  of  half-lapped  cotton  tape  six  mils  thick  on  the  end  con- 
nections to  protect  the  empire  cloth. 

(g)  The  coil  should  be  baked  and  impregnated  before  the  paper  and 
cotton  tape  are  put  on  and  dipped  in  finishing  varnish  after  they  are  all 
put  on  to  make  it  water  and  oil  proof. 

For  a  2200-volt  induction  motor  with  strip  copper  coils  and 
a  double-layer  winding: 

(a)  One  layer  of  half -lapped  cotton  tape  six  mils  thick  on  each  conduc- 
tor to  form  the  insulation  between  the  conductors. 

(6)  One  layer  of  half-lapped  cotton  tape  six  mils  thick  all  around  the 
coil  to  bind  the  conductors  together. 

(c)  One  turn  of  micanite  20  mils  thick  on  the  slot  part  of  the  coil. 

(d)  Two  layers  of  half -lapped  empire  cloth  six  mils  thick  all  around  the 
coil. 

(e)  One  turn  of  paper  10  mils  thick  on  the  slot  part  of  the  coil  to  protect 
the  empire  cloth. 

(/)  One  layer  of  half -lapped  cotton  tape  six  mils  thick  on  the  end  con- 
nections to  protect  the  empire  cloth. 

(g)  Bake  and  impregnate  the  coil  before  the  paper  and  the  last  taping 
of  cotton  tape  are  put  on.  After  that,  the  slot  part  of  the  coil  should  be 
hot  pressed  and  allowed  to  cool  under  pressure.  Then  the  coil  can  be 
dipped  in  finishing  varnish  to  make  it  water  and  oil  proof. 

Coil  and  Slot  Insulation  Employed  by  a  Large  Manu- 
facturer.— The  following  insulation  for  coils  and  slots  repre- 
sents the  practice  of  a  large  manufacturer  specializing  in  the 
construction  of  motors  and  generators  in  a  wide  range  of 
sbes  and  for  all  commercial  voltages. 

I.  For  Small  Direct-current  Machines  Using  Wire-wound  Coils: 

1.  Coils  are  wound  with  double  cotton-covered  wire. 

2.  Dipped  in  varnish  and  baked  to  about  100°C.  (212°F.)  until  coils 
are  free  from  stickiness. 

3.  Taped  with  6-mil  linen  tape. 

4.  Coils  are  then  placed  in  armature  and  soldered  to  commutator 
risers.     After  cleaning,  the  armature  is  dipped  or  sprayed  with 
varnish  and  baked. 

5.  The  final  operation  is  to  spray  with  shellac. 

6.  For  slot  insulation  10-mil  pressboard  is  used. 

II.  For  Engine  Type  Direct-current  Armatures  Using  Strip  Copper  Coils: 

1.  Copper  strips  are  taped  with  6-mil  linen  tape,  %  lapped.     Width 
of  linen  tape  is  %,  1  or  1  %  inch,  depending  upon  size  of  the  coil. 

2.  Coil  is  then  dipped  in  good  insulating  varnish  and  baked  until 
free  from  stickiness. 


INSULATING  COILS  AND  SLOTS 


169 


3.  When  coils  are  used  in  groups,  the  groups  are  taped  with  6-mil 
linen  tape  %  lapped. 

4.  Operation  two  is  then  repeated. 

5.  Coil  is  now  ready  to  be  inserted  in  armature  slots.     When  arma- 
ture is  completed,  and  bands  are  on  but  before  assembling  in 
frame,  the  windings  are  saturated  with  air-drying  varnish  or  it 
is  baked  until  free  from  moisture.     As  a  rule,  machine  is  well 
baked  by  running  under  full  load.     It  can  then  be  sprayed  with 
air  drying  varnish. 

6.  For  slot  cells,  10-mil  pressboard  is  used  not  so  much  for  insulation 
as  for  mechanical  protection. 


FIG.  121. — At  the  left,  armature  coil  insulated  for  a  250-volt  direct-current 
motor.  At  the  right,  stator  coil  insulated  for  a  220-volt  induction  motor 
(Crocker- Wheeler  Company}. 

Variation  from  above  practice  depends  upon  the  voltage.  For  high 
voltages,  say  550  volts,  the  coil  will  perhaps  be  taped  with  9-mil  oil 
muslin  half  lapped  and  an  additional  varnish  and  baking  treatment. 

For  mill  motors  the  coils  are  usually  insulated  with  flexible  mica.  The 
coil  is  heated  in  a  form  and  connected  in  the  secondary  of  a  transformer. 
While  the  current  passes  through  the  coil,  it  is  tightened  so  as  to  give 
the  proper  thickness,  alter  which  it  is  insulated  with  asbestos  tape. 
Asbestos  tape  is  used  because  of  the  severe  duty  to  which  these  machines 
are  subjected  and  because  they  are  completely  enclosed. 

III.  General  Method  for  Insulating  Direct-current  Field  Coils: 

1.  Coils  are  wound  with  double  cotton-covered  wire. 

2.  The  coil  is  then  taped  with  6-mil  linen  tape  about  %  lapped  on 
the  outside  of  coil.     Width  of  tape  one  inch. 

3.  Coils  are  next  placed  in  vacuum  tank  until  free  from  moisture. 


170          ARMATURE  WINDING  AND  MOTOR  REPAIR 

4.  Dipped  in  good  insulating  varnish  and  baked.  Or  if  the  varnish 
has  air  drying  qualities,  the  coils  are  dried  in  air  until  tree  from 
stickiness. 

IV.  Insulation  for  Stator  Coils  of  Alternating-current  Machines  for  600 
Volts  and  Under  Using  Wire-wound  Coils: 

1.  Coils  are  wound  with  double  cotton-covered  wire. 

2.  Heated  and  dipped  in  insulating  varnish  and  baked. 

3.  Operation  two  repeated. 

4.  Taped  with  6-mil  linen  tape  half  lapped. 

5.  Coil  is  then  shaped. 

6.  Dipped  in  varnish  and  baked. 

7.  Operation  six  repeated. 

8.  Operation  four  repeated. 

9.  Dipped  in   good   moisture-proof   varnish   and   baked   until   all 
crevices  are  well  filled. 

10.  For  slot  insulation  10-mil  pressboard  is  used. 

V.  Insulation  for  Stator  Coils  of  Alternating-current  Machines,  Made  up 
of  Strip  Copper: 

1.  One-turn  coils  are  taped  with  6-mil  linen  tape  one  inch  wide, 
half  lapped.     Two  turn  coils  are  taped  with  one  turn  as  above. 
Three  turn  coils  have  middle  turn  taped.     Four  turn  coils  have 
first  and  third  turns  taped. 

2.  Coil  is  then  taped  with  6-mil  linen  tape  half  lapped. 

3.  Dipped  in  insulating  varnish  and  baked. 

4.  Operations  seven,  eight  and  nine  as  for  wire  coils  are  then  applied. 

5.  For  slot  insulation  10-mil  pressboard  is  used. 

VI.  Insulation  for  Coils  Used  in  2200-volt  Machines: 

1.  Coil  is  wound  with  double  cotton-covered  wire. 

2.  Wrapped  with  6-mil  linen  tape  half  lapped. 

3.  Heated  and  dipped  in  insulating  varnish  and  baked. 

4.  Operation  three  repeated. 

5.  Operation  two  repeated. 

6.  Coil  is  then  formed. 

7.  Last  tape  is  now  removed  from  coil,  which  was  put  on  in  order 
to  protect  the  first  linen  tape  while  the  coil  was  being  shaped. 

8.  Wrapped  with  9-mil  oil  muslin  half  lapped  and  brushed  with  best 
quality  insulating  varnish. 

9.  Operation  eight  repeated. 

10.  Wrapped  with  6-mil  linen  tape  half  lapped. 

11.  Coil  is  then  dipped  in  insulating  varnish  and  baked. 

12.  Operation  ten  repeated. 

13.  Operation  eleven  repeated. 

14.  For  slot  insulation,  10  to  20-mil  pressboard  is  used  and  for  insu- 
lation between  layers  10-mil  pressboard. 

In  making  up  coils  for  any  kind  of  electrical  machinery,  the  main 
thing  to  bear  in  mind  is  to  apply  such  processes  as  to  eliminate  moisture 
from  the  coil.  Even  for  2200-volt  machines,  except  in  special  cases,  the 


INSULATING  COILS  AND  SLOTS 


171 


S     .S     «, 
CH       _^     C 


u  S  8 

•S  -o  8? 

I  M 

'S  •«  T3 


c    as  .a 

•*  a.| 


^     M   £ 

111 


ill 

•£     0  -2 

«H  > 

«?         || 

Jj  if 

s  •§! 

o   5  I 


s 


172         ARMATURE  WINDING  AND  MOTOR  REPAIR 

practice  of  the  company  employing  the  insulating  methods  described  is 
to  use  oil  muslin  and  depend  to  a  great  extent  upon  the  proper  varnish 
treatment.  After  much  experimenting  with  varnishes  it  has  been  found 
possible  to  wind  coils  that  will  withstand  high  voltages  equally  as  well  as 
insulated  coils  wound  for  the  same  voltages. 

Insulation  of  End  Connections  of  Coils.  In  a  double-layer 
winding,  the  voltage  between  the  end  connections  of  two 
coils  where  they  cross  each  other  at  the  ends  of  the  slot  may 
be  about  equal  to  the  terminal  voltage.  Suitable  insulation 
should  be  used  at  this  point.  A  belt  of  cotton  duct  is  usually 
used  between  the  end  connections  for  this  protection,  and  also 
to  protect  the  coils  from  mechanical  injury  by  rubbing  against 
each  other. 

Also  the  slot  insulation  should  be  allowed  to  extend  out  each 
end  of  the  slot  a  certain  distance  depending  upon  the  machine 
voltage.  Prof.  Alexander  Gray  gives  the  following  values  as 
typical  of  good  practice. 

Terminal  voltage  of  machine  Length  of   insulation  out  of  slot 

Not  over  800 0 . 75  inch 

800  to  2500 1 . 25  inches 

2500  to  5000 2.00  inches 

5000  to  7500 : 3 . 00  inches 

7500  to  11,000 4 . 50  inches 

Coils  for  alternating-current  windings  must  be  wound  to 
stand  the  line  voltage  to  ground.  In  a  Y-connecJied  machine 
the  total  insulation  from  copper  to  copper  between  two  coils 
should  stand  about  1.7  times  this  value.  The  first  coil  of 
each  phase  in  a  Y-connected  machine  has  a  voltage  equal  to 
the  line  voltage  to  ground  (volts  between  lines  -s-  \/3)  against 
which  to  insulate  and  should  be  given  a  protection  equal  to 
the  line  voltage.  The  first  and  last  coils  of  each  phase  group 
must  also  have  extra  insulation  for  protection  at  the  points 
where  they  cross.  These  are  called  the  phase  coils.  One 
layer  of  six-mil  empire  cloth  on  the  end  connections  covered 
with  six-mil  cotton  tape  half  lapped  in  addition  to  the  coil 
insulation  will  give  sufficient  insulation  to  the  phase  coils 
except  in  cases  of  voltages  past  2300.  (See  page  165  for 
phase  coil  insulation  for  2300  volts.) 


INSULATING  COILS  AND\8JbQTS  173 

Phase  Insulation  when  Reconnecting  fro.nl  Two-phase  to 
Three-phase  and  Vice  Versa. — In  reconnecting  the  winding 
of  an  induction  motor  from  two-phase  to  -  three-phase  or 
vice  versa  and  in  reconnecting  a  winding  for  a  different  number 
of  poles  to  change  the  speed,  it  is  necessary  to.  rearrange  the 
phase  insulation  because  the  spread  of  the  coils  per-phase, 
per-pole  is  being  changed. 

Mica  Insulation  for  Armature  Coils. — Mica  has  been  found 
to  be  a  first  class  insulating  material.  Its  insulation  re- 
sistance increases  with  temperature,  a  valuable  characteristic 
for  machines  operating  at  high  temperatures  and  in  direct 
contrast  with  the  properties  of  treated  tapes,  in  which  the 
insulation  resistance  and  loss  increases  rapidly  at  temperatures 
above  100°C.  or  212°F.  It  is  unaffected  by  temperatures 
far  in  excess  of  those  encountered  in  the  modern,  well-ven- 
tilated alternator.  It  is  also  impervious  to  the  static  dis- 
charges present  in  all  high  voltage  machines.  Furthermore, 
it  is  resilient  and  retains  its  resiliency  indefinitely — thus 
helping  to  hold  the  coil  tight  in  its  slot. 

Mica  is  a  mineral  obtained  in  the  form  of  large  crystals. 
These  split  readily  into  thin,  parallel-sided  laminae,  or  flakes. 
The  flakes  can  be  pasted  uniformly  on  cloth  or  paper  to 
facilitate  handling  and  to  provide  a  mechanical  support  dur- 
ing application.  In  the  form  of  a  "  wrapper/'  that  is,  pasted 
on  large  sheets  of  specially  treated  paper,  mica  is  mostly  used 
on  the  straight  sides  of  each  armature  coil,  to  provide  in- 
sulation between  conductor  and  iron,  the  operating  voltage 
of  the  machine  determining  the  number  of  turns,  or  the  thick- 
ness of  this  ineulation  wall. 

All  known  insulating  materials  are  relatively  poor  heat 
conductors.  This  is  equally  true  of  mica  and  treated  tapes. 
Therefore,  the  tighter  and  the  thinner  the  wall,  the  better  the 
heat  radiating  characteristics  of  the  coil.  For  the  lower 
voltage  machines  the  mica  wrapper  is  applied  as  tightly  as  is 
possible  by  hand.  For  the  higher  voltage  windings,  6600 
and  above,  where  the  insulation  wall  must  be  relatively  thick, 
special,  patented  machines  are  used  which  apply  the  wrapper 
under  heat  and  pressure,  and  finish  it  to  a  solid,  compact  wall. 

In   general,    all   of   the   larger   capacity   generators   have 


174 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


relatively  wide  cores.  Internal  "hot  spot"  temperatures, 
considerably  higher  than  those  measurable  by  thermometer, 
exist.  On  all  such  machines,  each  conductor  of  the  coil  is  also 
insulated  with  mica  tape. 

Repairing  Coils  Damaged  in  Winding  Process. — It  often 
occurs  that  the  insulation  of  a  coil  becomes  damaged  from 
charing  or  from  carelessness  in  the  use  of  tools  when  laying 
the  coils  in  the  .armature  slots.  Such  damage  should  be  re- 
paired at  once  to  prevent  possible  trouble  later  which  will 
make  the  repair  more  difficult  and  expensive.  To  repair 
a  damaged  coil  where  the  injury  to  it  is  only  slight,  the  coil 
should  be  removed  from  the  slot  and  all  the  insulation  removed 
from  the  injured  section.  Then  apply  an  overlapping  wrapper 


FIG.  123. — At  top,  one  whole  and  two  damaged  Eickemeyer  coils  taken 
from  an  elevator  motor.  At  bottom,  fiber  drifts,  slot  insulation,  hammer, 
parallel  jaw  pliers  and  coil  lifter. 

of  treated  cloth  and  around  this  a  protecting  covering  of  cot- 
ton tape.  Glue  down  the  ends  of  the  tape  securely  and  apply 
a  good  heavy  coat  of  shellac.  The  shellac  can  be  dried  quickly 
by  touching  a  lighted  match  to  it  when  the  alcohol  in  which 
it  is  dissolved  will  burn  with  a  blue  flame.  Care  must  be 
taken  that  the  tape  is  not  burned  in  doing  this.  In  such  a 
case  the  flame  will  turn  yellow  and  should  be  smothered  at 
once.  The  heat  of  the  burning  alcohol  is  not  usually  sufficient 
to  burn  the  tape.  After  this  the  coil  is  dry  and  can  be  put 
back  in  the  slot.  While  this  method  of  repair  can  be  safely 
used  in  small  armatures,  it  is  not  good  enough  for  large  arma- 
tures. In  the  latter  case  a  new  coil  should  be  used  or  the  old 
one  stripped  and  completely  reinsulated,  dipped  and  baked. 


INSULATING  COILS  AND  SLOTS 


175 


On  account  of  the  stiffness  of  the  insulation  on  the  terminals 
of  formed  coils,  before  connections  are  made  to  the  commutator 
it  is  a  good  plan  to  soften  the  leads  with  ^armalac"  or  a  similar 
armature  compound  at  the  points  where  the  leads  leave  the 
bottom  coils  in  the  slots.  This  will  prevent  breaks  while 
handling  the  leads. 

Voltage  to  use  when  Testing  Coil  and  Commutator  Insula- 
tion.— Under  certain  conditions  the  difference  of  potential 
between  coils  and  the  iron  core  of  a  machine  may  be  equal  to 
the  terminal  voltage.  Under  abnormal  operating  conditions 
it  may  even  be  greater.  It  is  important,  therefore,  that  the 
insulation  of  coils  and  commutator  shall  be  sufficient  to  stand 
a  voltage  considerable  larger  than  the  terminal  voltage  of  the 
machine  without  developing  grounds.  For  this  reason  a 
high-voltage  test  is  made  on  armature  windings  of  both  direct- 
current  and  alternating-current  machines  and  the  commutator 
of  the  former  before  the  winding  is  finally  completed. 

The  following  values  of  test  voltages  that  should  be  applied 
are  given  by  Prof.  Alexander  Gray  (Electrical  Machine  Design, 
page  34)  based  upon  the  standardization  rules  of  the  American 
Institute  of  Electrical  Engineers. 


Rated  terminal  voltage 
of  machine 

Rated  output  of  machine 

Testing  voltage 

Up  to    400 

Under  10  kw. 

1000 

Up  to    400 

10  kw.  and  over 

1500 

400  to    800 

Under  10  kw. 

1500 

400  to    800 

10  kw.  and  over 

2000 

800  to  1200 

Any 

3500 

1200  to  2500 

Any 

5000 

2500  and  over. 

Any 

Double-rated  voltage. 

Some  insulating  materials  will  withstand  very  high  voltages 
before  used  on  coils.  For  instance,  a  good  quality  of  oiled 
muslin  will  withstand  as  high  a  test  voltage  as  1500  to  2000 
volts  per  mil  for  9-mil  thickness,  however,  when  applied  to 
coils,  its  insulating  properties  will  diminish  because  of  handling. 
This  is,  of  course,  the  reason  why  so  much  insulation  is  used 
on  coils,  to  protect  them  from  becoming  grounded.  For 


176         ARMATURE  WINDING  AND  MOTOR  REPAIR 

coils  that  are  to  be  used  for  machines  below  2200  volts,  no 
steps  are  usually  taken  to  test  coils  before  placing  them  in 
the  machine.  However,  for  voltages  over  2200,  the  coils  are 
tested  before  they  are  placed  in  the  slots.  For  instance,  coils 
that  are  to  be  used  on  machines  of  6600  volts,  are  tested  for 
ground  at  20,000  volts  by  wrapping  them  with  tin  foil  before 
placing  them  in  the  slots.  Then  they  are  tested  with  15,000 
to  16,000  volts  after  being  placed  in  the  slots.  Finally  a  test 
of  at  least  twice  normal  voltage  is  made  for  one  minute. 

Field  Coil  Insulation. — For  the  insulation  of  field  coils, 
10  mil  paper,  J^g  inch  cardboard  and  6  mil  tape  can  be  used 
where  the  field  coil  has  a  cardboard  spool.  The  insulation 
can  be  applied  in  the  order  named  using  2  layers  of  tape  and 
paper  with  the  paper  next  to  the  wires  on  the  spool.  The 
coils  should  then  be  baked  in  a  vacuum  tank  and  impregnated. 

Varnishes  and  Impregnating  Compounds  for  Coils. — The 
varnish  used  over  the  outside  of  insulated  coils  should  be 
water,  oil  and  acid  proof  and  dry  quickly  in  air  forming  a  hard 
smooth  surface.  Such  varnishes  are  made  by  a  number  of 
manufacturers. 

Compounds  for  use  in  impregnating  coils  are  also  available 
in  the  open  market  and  are  usually  an  asphaltum  or  a  paraffin 
base  dissolved  in  a  suitable  thinning  solution.  It  is  important 
that  the  material  will  not  attack  copper,  iron  or  insulating 
materials  used  and  form  a  solid  at  all  temperatures  below 
212°F.  without  contraction  when  changing  from  the  fluid  to 
the  solid  state.  This  material  should  not  be  applied  at 
temperatures  above  the  breakdown  of  cotton  materials, 
that  is,  above  a  temperature  of  248°F. 

Because  the  desirable  characteristics  for  a  perfect  varnish 
cannot  be  combined  into  one  compound,  a  number  of  varnishes 
have  been  developed,  each  having  its  own  characteristic. 
The  purposes  of  some  insulating  varnishes  of  the  Sherwin- 
Williams  company  are  indicated  in  the  accompanying  table. 
Where  more  than  one  varnish  can  be  successfully  used  the 
different  types  are  indicated  as  first,  second  and  third  choices. 

All  of  the  varnishes  mentioned  in  the  table  are  of  the 
baking  type.  However,  insulating  varnishes  in  general 
may  be  divided  into  several  general  classes,  such  as  clear  var- 


INSULATING  COILS  AND  SLOTS 


177 


nishes,  black  varnishes,  baking  varnishes  and  air-drying 
varnishes.  The  most  marked  difference  between  the  clear 
and  the  black  varnishes  is  the  color,  but  owing  to  fundamental 
differences  in  the  characteristics  of  the  ingredients  entering 
into  their  composition,  there  are  also  some  differences  in  the 
physical  properties  of  the  varnishes  themselves.  As  a  general 
rule  clear  varnishes  possess  greater  mechanical  strength  and 
resist  oil  better  than  black  varnishes.  An  exception  to  this  is 
the  black  elastic  baking  varnish  shown  in  the  table.  Where 
extreme  mechanical  strength  is  required  as  on  small  high-speed 
armatures  clear  varnishes  are  almost  always  used. 

TABLE  SHOWING  SUITABILITY  OF  INSULATING  VARNISHES 


Characteristics  of  clear  and  black 
baking  insulating  varnishes  and 
uses  for  which  they  are 
recommended  * 

Clear  varnishes 

Black  varnishes 

Clear 
quick 
baking 

Clear 
quick 
elastic 
baking 

Clear 
elastic 
baking 

Black 
quick 
baking 

High 
Heat 
resist- 
ing 
baking 

Black 

plastic 
baking 

Black 
elastic 
baking 

Dielectric   strength  
Mechanical  strength  

3 
3 
3 

1 
2 

1 
2 
3 

2 

1 
1 
1 
2 
2 
1 
1 
1 

1 
1 
1 

1 
2 

1 
2 
1 

1 
1 
2 

2 
2 
2 
1 

1 

1 

2 
2 
2 
2 
2 

3 
3 
3 

2 
2 
4 

3 

2 
3 
3 
3 
3 

1 
3 

2 
4 
3 

3 
2 

2 

3 
3 

3 
2 
2 

1 
2 

2 

2 
5 
2 
1 
4 
1 
1 

2 
3 

3 
2 
1 

1 
1 

1 
2 

1 

1 
2 
1 

2 
2 
2 
1 
1 

1 

2 
2 
2 
2 
2 

Flexibility  

Plasticity 

Oil  resistance  

2 
2 

4 

3 

2 
3 
3 
3 
3 

1 

Water  resistance  

Treating  cloth,  paper  and  thin  fib- 
rous materials  
Treating    fullerboard     and    heavy 
fibrous  materials  

Small  high-speed  armatures  
Intermediate-speed  armatures  
Large  low-speed  armatures  
Field  and  stator  coils  

Automobile-starting  motors  
Vacuum-cleaner  motors  

Washing-machine  motors 

Street  railway  and  electric  locomo- 
tive motors 

Fan  motors 

3 

Magnetos  and  induction  coils  
High-potential   apparatus 

Transformers 

Average  repair  shop  conditions.  .  . 

*  Numbers  indicate  order  of  suitability. 

Black    varnishes    are    not    quite    so    strong    mechanically 
as  clear  varnishes  but  are  sufficiently  strong  for  most  purposes. 


178         ARMATURE  WINDING  AND  MOTOR  REPAIR 

On  stationary  windings,  as  on  alternating-current  stator 
windings,  the  varnish  is  not  subjected  to  centrifugal  stresses 
and  there  is  no  advantage  in  using  a  clear  varnish.  Certain 
black  varnishes  are  made  from  plastic  materials  and  have  the 
ability  to  withstand  long-continued  heating  without  hardening. 
Black  varnishes  as  a  rule  are  cheaper  than  clear  varnishes  and 
are  more  commonly  used  for  that  reason. 

The  chief  difference  between  baking  varnishes  and  air- 
drying  varnishes  is  in  the  proportion  of  oxidizing  ingredients 
contained.  The  baking  varnishes  are  tougher,  more  elastic, 
more  resistant  to  oil  and  water,  and  have  longer  life  under 
heat.  Speed  in  drying  is  always  accomplished  at  the  expense 
of  these  characteristics,  and  the  air-drying  varnishes  are  less 
durable  and  elastic  than  the  baking  varnishes.  The  air- 
drying  varnishes  find  their  principal  field  of  usefulness  on 
apparatus  where  severe  conditions  of  usage  are  not  encountered 
and  for  quick  repair  work. 

Characteristics  of  Insulating  Varnishes. — An  important 
factor  concerning  insulating  varnishes  is  that  the  dielectric 
resistance  increases  directly  with  the  length  of  baking  or  dry- 
ing period,  the  slow  varnishes  imparting  the  highest  degree  of 
insulation  and  flexibility  and  producing  a  tough,  flexible  film 
which  may  also  be  depended  upon  for  mechanical  strength  and 
extreme  durability.  Black  varnishes  are  claimed  to  be 
better  for  work  where  a  transparent  coating  is  not  absolutely 
essential  because  they  produce  a  more  flexible  and  highly 
insulating  film  than  clear  varnishes  of  the  same  class.  Var- 
nishes should  not  crystallize  under  prolonged  vibration,  and 
their  mechanical  structure  should  be  elastic  and  homogeneous. 
The  accompanying  table  (page  179)  gives  the  characteristics, 
uses,  drying  time  and  solvents  of  several  varnishes. 

Solvent  Chart  for  Insulating  Varnishes. — Since  varnish  is 
usually  sold  in  concentrated  form  care  must  be  taken  in 
dissolving  to  avoid  wrinkles  or  stringy  drip  forming  on  coils 
when  varnish  is  too  heavy.  Benzine  is  preferable  to  gasoline 
as  a  solvent  but  gasoline  can  be  used.  The  solvent  and  varnish 
should  be  approximately  the  same  temperature  and  neither 
should  be  under  60°F.  (15.5CC.).  The  chart  on  page  180 
shows  the  percentage  of  58  degrees  benzine  to  be  added  to  every 


INSULATING  COILS  AND  SLOTS 


170 


•3 

•a 

a 

| 

Solvent 

0- 
G 

0> 

a 

J 

| 
1 

1 

| 

"ol 

1 

1 

1 

1 
.a 

G 

a 

'1 

a 

1 

1 

a  ^ 

« 

1 

9 

CP 

CD 

Q 

IV 

Q 

Is 

IS 

3d* 

<N 

1C 

bi 

bi 

bi 

bi 

MU3 

bio 

III 

W 

^ 

< 

<5 

< 

< 

<    N 

<J  N 

IB 

00 

o 

00 

^1 

*v  ° 

3 

^ 

4" 

J 

£ 

M 

2cS 

s^ 

^,  ^ 

Jj    rH 

H 

^—~-s 

^^^-> 

§ 

CO     "t^ 

11 

resisting 

resisting 

b£ 

a 

durable  . 

1 
"I 

resisting 

11 

I|   : 

"2    • 

'i 

11 

1 

Ii 

1 

1 
I 

11 

t?  i 

05      _ 

|| 

ii 

S- 

bO 

3 

c«  .2 
ll 

1 

S 

11 

"5   ft    ! 

Prevents  ab 
ure,  resists 
and  acid.  . 

|1 

^ 

ii 

|1 

Oilproof,  to 

Water,  oil  a 
Heat  cond 

Waterproof 

ft    : 

a     • 

a 

a 

03 

ft  ; 

2  g 

72 

£ 

1 

• 

-  "05 

a 

tn 

y 

fl 

a 

9 

G 
IS 

a 

03 

"*^     <D 

S    ro 

£ 

(-1 

'g 

§ 

£ 

1 

a  03 

0     « 

—    a? 

"S 

§ 

to 

03 

S 

3 

a 

£     3 

JS    o 

i 

*o3 

i 

73 

o 

S   fl" 

S* 

a) 

£  o 

o 

03      Jj 

^.2 

bfi 

1 

W 

i 

a3  '^3 
03 

03    aS 

a  ,zf 

1  1 

a 

| 

"o 

S 
6 

i 

•g  | 

tt  >FH 

*3 

s 

03 

j  . 

CH       ^ 

o  'S 

*  1 

G 

0> 

bJO 

G 

•3  £ 

>    <a 

fl 

a   £ 

o   | 

^H      ® 

J 

3 

JS 

g 

.22 
G 

0 

"o 

i| 

|i 

I 

1 

i 

'3 

; 

ii 

'a 

q 

i 

'a 

1 

M 

> 

n 

q 

bO 

a 

*M 

1 

_g 

T3 

bfi 

a 
la 

a3 
1 

1 

9 

03 

a 

J 

1 

1 

'3 

'3 
ya 

a 

g 

o 

1 

! 

J 

M 

i 

1 

1 

1 

O 

pq 

O 

5 

0 

0 

S 

180 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


CO 

»  5 

d 

JS                               o 
2   «    a                  cs   o> 
n  '43  a               9  *s 

d 

0  •-     0                       05     g 

ivf*             -j  02     • 

f-     CX                                ^ 

i;i      11 

'P 

fc 

la 

0 

1  !  »       ".  ™      °  "  2 

DOMPA 

w$3 

o 

II?      ^     °"" 

e 

1 

CO 

d 

g  M  pq 

FACTU 

05 
2>S 

O 

So^M                               OiOOJ'O'-HOO 
3           00                                                     i-<    (N    Cl 

o^   IY 

p 
fc 

•* 

10        , 
00    CO 

d 

tigs 

W 

— 

P 

1 

*£ 

0 

•S§S|             o^^.e^ 
.-  1  ^    1    ° 

w 

73 

m 

V 

y3 

is 

o 

§|^|                 X» 
J|  13                °'*oc22^°co^ 
-^  **  >.  S 

a 

DO 

§ 

i 

1 

o 

°°  ra 

o 

•I|   S  6  b  •*  00  5  5"  w  ?  «•  « 

J     g     60                               ^-<<N<NCO^iO 

0 

0 

IO 

t*  o 

d 

^                                                                                                                                V<N 

73 

Ir 

d 

*g^-s«ai|:« 

fc 

— 

2 

CD 

oo 

00    N 

d 

\IN             \IN    \N 

OcOOO-iiOOiOOCOCOOOOI^ 
^i-i(N(NCOCOrJ(iOiOtD 

2J 

w 

»  w 

o 

^-;2f^^g3K 

o 

d 

^v        -^\  -\ 

1 

CO 

§M 

o 

-A, 

0 

SSS5SSSSSSS8SSSJ5S8S 

1      1 

OJOOOOGOOOOOOOOOOOoroOOOGOOOGOGO 

dddddddddddddddd 

•^TABJO  ogioadg  pajisaQ 

INSULATING  COILS  AND  SLOTS  181 

100  gallons  of  insulating  varnish  to  bring  it  to  the  correct 
specific  gravity.  The  most  suitable  gravity  should  be  estab- 
lished by  trial  and  the  specific  gravity  kept  constant  at  this 
value.  With  63  degrees  gasoline  15  per  cent,  less  should  be  used 
than  that  shown  and  with  54  degrees  benzine  10  per  cent,  more 
should  be  added. 

Method  for  Making  Tape  from  Cotton  Cloth. — Maurice 
S.  Clement  has  described  the  following  method  (Electrical 
Record,  October,  1918)  for  making  tape  as  used  by  a  middle 
western  electrical  repair  shop  owing  to  a  shipment  of  cotton 
tape  being  held  up  on  account  of  war  conditions.  The 
repair  job  was  urgent  and  the  repairman  was  forced  to 
make  up  an  amount  of  cotton  tape  of  different  widths  rang- 
ing from  J-^  inch  to  2  inches,  sufficient  to  keep  its  main- 
tenance men  going  until  the  order  arrived.  After  considerable 
thought,  the  following  was  decided  upon  as  a  temporary 
remedy. 

A  bolt  of  white  cotton  cloth  was  purchased  in  a  nearby  dry 
goods  store.  The  cloth  was  rolled  tightly  on  a  half -inch  dowel 
and  then  marked  off  to  the  various  widths  on  the  outside.  A 
band-saw  was  used  to  good  advantage  cutting  the  cotton  cloth 
into  tape;  as  this  tape  had  no  salvage  edge,  steps  had  to  be 
taken  to  prevent  it  from  unravelling.  Both  edges  of  each 
roll  of  tape  were  given  a  heavy  coat  of  shellac  and  before  it 
dried  the  alcohol  was  burned  off.  This  has  a  sort  of  semi- 
baking  effect  which  tends  to  strengthen  the  tape.  It  also 
prevents  the  tape  from  curling. 

Drying  Out  Insulation  of  Direct-current  Generators  (In- 
struction Book,  Westinghouse  Electric  &  Mfg.  Co.). — Drive 
the  generator  by  a  motor  connected  by  a  belt  and  short- 
circuit  the  armature  beyond  the  ammeter  using  a  very  weak 
field  excitation.  If  the  generator  is  shunt  wound,  low  voltage 
separate  excitation  must  be  employed;  if  compound  wound 
the  armature  may  be  short-circuited  through  the  series  field 
coifs.  Direct  -  current  generators  are  very  sensitive  when 
operated  as  series  machine  and  there  is  danger  of  generating 
an  excessive  current.  Consequently  this  method  should  be 
undertaken  only  by  experienced  operators. 

The  field  coils  may  be  dried  by  applying  from  some  separate 


182          ARMATURE  WINDING  AND  MOTOR  REPAIR 

source  of  excitation  approximately  two-thirds  of  the  normal 
direct-current  voltage. 

There  is  always  danger  of  serious  injury  to  the  windings 
when  drying  out  with  current  since  the  heat  generated  in  the 
inner  parts  is  not  readily  dissipated;  furthermore,  coils  con- 
taining moisture  are  much  more  susceptible  to  injury  from 
overheating  than  when  thoroughly  dry.  The  temperature 
of  all  accessible  parts  should  be  carefully  observed  during 
the  drying  out  process  and  never  allowed  to  exceed  80°C. 
(176°F.),  total  temperature.  Several  hours  or  even  days 
may  be  required  for  thoroughly  drying  out  large  machines. 
During  the  drying  out  process  the  temperature  should 
not  be  allowed  to  drop  below  that  of  the  surrounding 
air  as  moisture  then  condenses  on  the  coil  surfaces  and  the 
effect  of  the  previous  drying  would  be  largely  lost.  At 
regular  intervals  during  the  drying  out  run,  readings  of  the 
insulation  resistance  (see  page  185)  may  be  taken  at  regular 
intervals  and  plotted  as  a  curve,  using  time  for  the  horizontal 
scale  and  resistance  for  the  vertical  scale.  The  drying  should 
continue  until  the  resistance  has  begun  to  increase.  If  the 
insulation  contains  appreciable  moisture  the  resistance  will 
decrease  during  the  first  part  of  the  drying  out  process. 

Heating  windings  by  current  is  more  effective  than  any 
process  of  heating  from  the  outside,  such  as  enclosing  the 
machine  and  heating  the  air  by  resistance  or  fires,  because  in 
the  former  method  the  inside  of  the  coils  becomes  hotter  than 
the  outside  and  .moisture  is  driven  outward.  With  external 
heating  the  reverse  is  true. 

Drying  Out  Synchronous  Motors  and  Generators. — Syn- 
chronous motors  and  generators  can  be  dried  out  by  rotat- 
ing the  motor  or  generator  at  any  convenient  speed  and  short- 
circuiting  the  armature  beyond  the  ammeters.  The  field 
should  be  excited  so  that  the  desired  heating  current  will  flow 
in  the  armature  winding.  For  windings  of  2400  volts  or  lower, 
the  temperature,  as  measured  by  thermometers  properly 
applied  to  the  hottest  accessible  part  of  the  winding  should  not 
be  higher  than  80°C.  (176°F.).  For  6600-volt  windings  the 
temperature  should  not  be  higher  than  75°C.  (167°F.)  and  for 
11,000  to  13,200-volt  windings  not  higher  than  65°C.  (149°F.). 


INSULATING  COILS  AND  SLOTS  183 

The  reason  for  specifying  the  lower  temperature  for  the  higher 
voltage  windings  is  the  greater  difference  in  temperature  be- 
tween the  inside  of  the  coil  and  the  outside  (when  the  tem- 
perature is  measured  with  a  thermometer)  in  the  coils  having 
the  thicker  insulation. 

If  a  low  voltage  (5  to  15  per  cent,  of  normal)  can  be  obtained 
from  the  taps  on  a  transformer  for  example,  the  armature 
winding  can  be  dried  out  by  applying  this  low  voltage  to  the 
armature  terminals,  the  rotor  remaining  stationary.  The 
field  winding  should  be  short-circuited  and  the  temperature 
of  the  cage  winding  on  the  rotor  should  be  watched.  Less 
than  normal  current  will  be  necessary  on  account  of  the 
absence  of  ventilation. 

For  medium  sized  alternators  and  synchronous  motors  a 
satisfactory  way  of  drying  out  both  field  and  stator  windings 
is  to  connect  the  machine  to  an  alternating-current  circuit 
and  run  as  a  motor  with  fields  overexcited  at  zero  power  factor. 
This  method  is  both  cheap  and  effective. 

Drying  Out  Induction  Motors. — Small  motors  can  be  baked 
in  ovens.  The  temperature  should  be  raised  gradually  taking 
several  hours  to  bring  it  to  the  maximum  value  which  should 
not  be  more  than  80°C.  (176°F.)  at  the  hottest  point.  The 
temperature  should  be  maintained  constant  for  from  one  day 
to  a  week  depending  on  the  size  and  voltage  of  the  machine 
and  the  history  of  its  exposure  to  moisture.  Induction  motors 
can  also  be  dried  by  operation  at  no  load  on  low  voltage  (the 
primary  current  and  heating  increases  as  the  voltage  is  re- 
duced) or  by  a  still  lower  voltage  that  will  circulate  a  suffi- 
ciently heavy  current  with  the  rotor  blocked.  If  a  sufficiently 
low  alternating-current  voltage  is  not  available,  direct  current 
may  be  used.  There  is  always  more  or  less  danger  of  over- 
heating the  windings  of  a  machine  when  drying  them  with 
current  as  the  inner  parts  which  cannot  quickly  dissipate 
the  heat  generated  in  them  and  which  cannot  be  examined, 
may  get  dangerously  hot  while  the  exposed  and  more  easily 
cooled  portions  are  still  at  a  comparatively  moderate  tempera- 
ture. The  temperature  of  the  hottest  part  accessible  should 
be  measured  during  the  drying  out  process  and  not  allowed 
to  exceed  80°C.  (176°F.). 


184 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


Insulation  Test. — During  the  drying  out  of  a  machine, 
insulation  resistance  tests  should  be  made  at  regular  intervals 
and  plotted  in  the  form  of  a  curve  using  time  on  the  horizontal 
scale  and  values  of  insulation  resistance  on  the  vertical  scale. 
The  drying  out  should  be  continued  until  the  resistance 
reaches  it  proper  value.  The  insulation  resistance  is  at  best 
only  a  rough  guide  in  determining  the  condition  of  the  ma- 
chine as  to  moisture  and  relative  values  in  the  same  winding 
during  a  drying  out  run  are  of  more  value  than  the  relative 
values  of  windings  in  other  machines. 


500  to  600  Volt  D.C.  Circuit 

[£ 

^ 

1      2 

—  —  •      •  — 

L 

3     4 

Double  throw, 
double  pole  switch 


To  resistance  to 
be  measured 


Voltmeter 

FIG.  124. — Connections  of  double-pole,  double  throw  switch  and  500-volt 
v      voltmeter  for  measuring  insulation  resistance. 

A  megger  is  also  sometimes  used  for  testing  the  condition 
of  the  windings  during  the  drying  out  prcoess. 

The  insulation  resistance  of  a  machine  in  good  condition 
and  at  its  operating  temperature  will  usually  not  be  less  than 
the  value  given  by  the  following  formula: 

T      ,    .  .  .  Machine  voltage 

Insulation   resistance   in   megohms  =  ^   .    ,  T^ — 

Rated  Kva.  +  1000 

For  example  a  1000-Kva.,  11,000-volt  motor  should  have  an 
insulation  resistance,  if  clean  and  dry,  of  5.5  megohms. 
The  insulation  resistance  of  field  windings  will,  in  general,  be 
much  higher  in  proportion  to  the  operating  voltage  than  that 
of  the  armature.  Since  large  armatures  have  much  greater 
areas  of  insulation,  their  insulation  resistance  will  be  propor- 
tionately lower  than  that  for  small  machines.  The  insulation 


INSULATING  COILS  AND  SLOTS  185 

resistance  of  any  machine  will  also  be  much  lower  when  hot 
than  when  cold,  especially  when  the  machine  is  heated  rapidly. 
Measuring  Insulation  Resistance. — Insulation  resistance 
may  be  measured  with  a  megger  or  by  the  use  of  a  500-volt 
direct-current  voltmeter  and  a  500  volt-direct-current  circuit. 
Connect  the  voltmeter  as  shown  in  Fig.  124  and  read  first,  the 
voltage  of  the  line;  then  connect  the  resistance  to  be  measured 
by  throwing  the  double  throw  switch  and  read  the  voltmeter  a 
second  time.  The  insulation  resistance  is  then  calculated  by 
the  following  formula: 

Insulation  resistance  (R)  =  ,  ^  MOfrOOO        '   '; 

Where  V  is  voltage  of  the  line;  v  the  voltage  reading  with 
insulation  in  series  with  the  voltmeter;  r  the  resistance  of 
the  voltmeter  in  ohms  which  is  generally  marked  inside  the 
instrument  cover,  and  R  the  resistance  in  megohms.  A  meg- 
ohm is  equal  to  one  million  ohms. 

If  a  grounded  circuit  is  used  in  making  the  measurement, 
care*must  be  taken  to  connect  the  grounded  side  of  the  line  to 
the  frame  of  the  machine  to  be  measured  and  the  voltmeter 
between  the  windings  and  the  other  side  of  the  circuit. 


CHAPTER  VIII 

REPAIR  SHOP  METHODS  FOR  REWINDING  ALTER- 
NATING-CURRENT MACHINES 

I.  WINDING  SMALL  SINGLE-PHASE  MOTORS 

The  complete  stator  winding  of  most  small  single-phase 
induction  motors  is  made  up  of  two  windings;  the  main  wind- 
ing of  many  turns  of  heavy  wire  and  what  is  known  as  the 
"teaser"  or  starting  winding.  The  latter  is  necessary  because 
a  single-phase  motor  is  not  self -starting  and,  therefore,  requires 
some  means  of  producing  a  rotating  field  to  overcome  this 
deficiency.  This  the  starting  winding  does.  It  is  of  smaller 
wire  than  the  main  winding  and  of  high  resistance. 

The  method  of  winding  single-phase  motors  differs  from^that 
used  for  other  alternating-current  motors  in  that  a  skein 
winding  is  often  used.  That  is,  the  winding  coil  is  in  the 
form  of  a  skein  of  wire  which  is  looped  many  times  through 
several  slots  to  form  a  pole  of  the  winding.  The  details  of 
this  method  of  winding  as  given  in  what  follows,  are  based 
on  articles  that  have  appeared  in  the  Electric  Journal  by 
G.  I.  Stadeker  and  C.  A.  M.  Weber. 

Insulating  Lining  for  Slots. — The  slots  should  be  lined  with  a 
protecting  cell  of  fish  paper  cut  to  fit  the  slot.  Inside  this  a 
cell  of  treated  cloth  should  be  placed  cut  so  that  its  edges  will 
project  about  %  inch  beyond  the  entrance  to  the  slot.  In 
those  slots  which  will  contain  both  the  main  and  the  starting 
winding,  an  extra  treated  cloth  cell  should  be  inserted  over 
the  main  winding  to  enclose  the  starting  coils.  End  plates  of 
fullerboard  or  fiber  are  used  to  insulate  the  core  from  the 
windings.  The  end  connections  of  the  main  and  starting 
windings  should  be  separated  by  friction  cloth. 

Winding  the  Skein  Coil. — In  repairing  a  motor,  the  number 
of  times  the  skein  is  to  be  looped  through  the  slots  and  .the 
length  of  the  skein  can  best  be  obtained  from  a  skein  taken 

186 


REWINDING  ALTERNATING-CURRENT  MACHINES    187 

from  the  burned-out  machine.  In  removing  the  old  winding 
care  should  be  taken  to  preserve  one  entire  skein  if  possible. 
If  this  is  impossible  a  satisfactory  scheme  for  the  repairman 
is  by  trial  with  a  single  wire.  This  wire  should  be  laid  in  the 
slots  exactly  as  the  skeins  of  wire  will  be  laid,  proper  allowance 
being  made  for  the  building  up  of  the  skein  ends  from  slot  to 
slot.  The  wire  should  then  be  removed  and  measured.  Make 
up  a  trial  skein  of  this  length  and  wind  it  in  the  slots.  Cor- 
rections if  necessary  can  be  made  on  the  next  skeins  made  up. 
Inserting  the  Skein  Coil  in  the  Slots. — After  the  skein  length 
and  distribution  have  been  obtained  from  the  old  motor,  the 


abode  f  g 

FIG.  125. — Successive  steps  in  applying  a  skein  coil  for  main  winding  of  a 
split-phase,  GO-cycle,  4-pole,  24-slot  induction  motor. 

exact  procedure  in  winding  is  as  shown  in  Fig.  125  (a)  to  (g). 
The  distribution  of  a  24-slot  primary  winding,  indicating  the 
number  of  times  the  main  winding  skein  is  wound  into  each  slot, 
is  shown  in  Figs.  126  to  129.  The  distribution  in  Fig.  126  is 
one  commonly  used.  Other  distributions* may  be  used,  but  in 
all  skein  windings  the  wires  in  any  slot  must  be  a  multiple  of 
the  wires  in  the  skein. 

A  developed  view  of  the  primary  or  stator  winding  is  shown 
in  Fig.  (125a),  looking  at  the  teeth  with  the  first  operation  of 
putting  the  skein  winding  in  slots  3  and  5  completed.  The 
end  of  the  coil  thus  formed  should  be  firmly  pressed  against 
the  core,  using  a  rawhide  or  fiber  mallet  or  a  piece  of  smooth 
wood.  A  half  twist  is  next  made  in  the  skein,  as  shown  in 


188 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


Fig.  125  (6),  and  the  loop  laid  back  over  the  winding  and 
threaded  into  slots  2  and  6,  as  in  Fig.  125  (c).  The  half  twist 
is  repeated,  as  in  Fig.  125  (d),  and  the  loop  laid  back  in  slots  2 
and  6  for  the  second  time,  as  in  Fig.  125  (e).  This  second 
half  twist,  in  Fig.  125  (d),  must  be  in  the  opposite  direction  to 
the  first  one  (Fig.  125  (6)),  to  bring  the  same  side  of  the  loop  on 
top.  Otherwise  a  twist  will  be  put  in  the  skeins,  which  will 


Slot  Number 
Main  Winding 

1 

2 

3 

4 

fi 

8 

7 

8 

9 

10 

11 

12 

13 

U 

15 

16 

17 

18 

19 

20 

21 

22 

23  24 

Distribution 

1 

1 

-~ 

i 

i 

1 

1 

! 

i 

1 

i 

! 

1 

! 

1     1 

Starting  Winding 
Distribution      . 

1 

1 

] 

1 

1 

1 

1 

1 

J 

L 

FIG.  126. — Distribution  of  main  and  starting  winding  coils  of  a  4-pole  motor 
showing  skeins  overlapped. 


Slot  Number          1    23     i    5    6    78    9    10  11  12  13  14  16  16  17  18  19  20  21  22  23  24 


Main  Winding 
.  Distribution 


Starting  Winding 
Distribution 


FIG.  127. — Distribution  to  avoid  overlapping  of  skeins. 


Slot  Number 

1 

2 

9 

t 

r> 

6 

7 

i 

9 

10 

11 

12 

13 

11 

15 

16 

17 

18 

19 

20 

21J22 

23  24 

Distribution 

1.1 

1 

i 

i 

! 

i. 

•_• 

1 

1 

2 

,. 

| 

1 

1 

•> 

1.1 

1 

T. 

1     2 

Starting  Winding 
Distribution 

i 

1.1 

i 

1.1 

1 

I 

1.1 

& 

i 

2 

FIG.  128. — Distribution  of  an  8-pole  machine. 


Slot  Number          1234     5    6    7    8    9    10  11  12  13  14  15  16  17  18  19  20  21  22  23  24 


Main  Winding 
Distribution 


Starting  Winding 
Distribution 


FIG.  129. — Distribution  of  an  8-pole  consequent  wound  machine. 

make  it  hard  to  winfl  smoothly,  especially  if  it  is  looped  back 
and  forth  many  times.  The  half  twist  in  Fig.  125  (/)  is  made 
in  the  same  direction  as  that  in  Fig.  125  (6)  By  looping  the 
turns  into  slots  1  and  7  the  winding  of  the  skein  is  completed. 
The  winding  of  the  second  and  subsequent  coils  is  exactly  the 
same  as  the  first.  The  completed  winding  for  a  four-pole, 
24-slot  machine  is  shown  in  Fig.  130  (at  top). 

In  split-phase  starting,  squirrel-cage  motors  the  starting 
winding  is  connected  across  the  line  until  approximately  two- 


REWINDING  ALTERNATING-CURRENT  MACHINES     189 


thirds  synchronous  speed  is  reached,  when  its  circuit  is  auto- 
matically opened  by  a  centrifugally-operated  switch.  The 
center  of  the  starting  winding  is  between  the  pole  centers  of 


FIG.  130. — At  top,  complete  main  winding  for  a  4-pole,  60-cycle,  split- 
phase  induction  motor.  At  bottom,  complete  main  and  starting  winding  for 
the  same  motor. 

the  main  winding.  Its  distribution  and  the  length  of  the 
skeins  must  be  determined  by  trial  as  explained  for  the  main 
winding.  The  starting  winding  is  a  resistance  winding,  con- 
sequently it  is  very  important  that  its  re?istance  in  the 


FIG.  130  (a). — Dissembled  view  of  a  single-phase  repulsion  induction  motor. 

motor  be  the  same  as  it  was  originally,  which  makes  the  length 
of  the  skein  important.  The  distribution  of  both  the  main 
and  starting  windings  and  the  number  of  times  the  skein  is  to 


190         ARMATURE  WINDING  AND  MOTOR  REPAIR 


be  wound  into  each  slot  are  shown  in  Fig.  130  (lower  diagram). 

After  all  the  slots  have  been  filled  the  coils  should  be  forced 
into  position  with  a  fiber  drift,  the  insulating  cells  folded  in 
and  fiber  wedges  driven  in  the  slots. 

Winding  for  a  Repulsion- start  Motor. — For  repulsion- 
starting,  induction-running  motors  the  primary  winding  is 


1234 

FIG.  131  (a),  (6),  (c).  —  At  left,  primary  connections  for  a  single-phase, 
repulsion-starting  induction  motor.  In  center,  connections  for  a  4-pole, 
split-phase  motor.  At  right,  same  as  in  center  but  for  a  series-parallel 
connection  of  coils. 

In  the  diagram  at  the  left,  N  denotes  neutral  points.  For  a  220-  volt  circuit  leads  2  and 
3  should  be  connected  together  and  leads  1  and  4  to  the  line.  For  a  110-volt  circuit 
leads  1  and  2  should  be  connected  in  parallel  to  one  line  and  leads  3  and  4  to  the  other. 
In  the  center  diagram  the  coils  are  connected  in  series.  To  obtain  clockwise  rotation, 
leads  1  and  2  should  be  connected  to  one  line  and  leads  3  and  4  to  the  other.  For  counter 
clockwise  rotation,  leads  1  and  3  should  be  connected  to  one  line  and  leads  2  and  4  to  the 
other. 

complete,  as  shown  in  Fig.  130,  and  the  coils  are  connected 
together,  as  indicated  in  Fig.  131  (a).  Four  leads  arc  brought 
out,  so  that  these  motors  may  be  connected  externally  for 
either  220  or  110  volts. 

Winding  Small  Motors 
by  Hand  .  —  A  method 
sometimes  employed  in 
winding  small  motors  of 
the  induction  type  is 
known  as  hand-winding. 

FIG.  132.  —  Hand-winding  method  for  main     The      Conductors     are 
coils  of  primary  shown  in  Fig.  130. 


at  a  time,  beginning  at  the  center  of  a  pole,  as  shown  in 
Fig.  132.  This  method  is  mostly  used  when  the  number  of 
turns  in  the  slots  have  no  relatively  large  common  divisor, 
thereby  eliminating  the  skein-winding  method.  The  hand- 
winding  method  should  not  be  employed  for  the  starting 


REWINDING  ALTERNATING-CURRENT  MACHINES     191 

winding  of  a  split-phase  motor,  because  the  resistance  of  the 
starting  winding  would  not  always  be  that  required.  A 
skein  winding  should  be  used. 

Windings  for  Odd  Frequencies. — It  frequently  happens 
that  small  motors  must  be  wound  for  odd  frequencies,  such  as 
125,  133,  and  140  cycles  on  standard  60-cycle  motor  cores. 
The  number  of  poles  of  such  motors  is  usually  large,  resulting 
in  a  small  number  of  slots  per  pole.  Using  a  24-slot  primary, 
wound  eight  poles,  the  slots  per  pole  would  be  three  and  the  dis- 
tribution of  the  winding  would  be  as  shown  in  Fig.  128.  Such 
a  winding  is  very  difficult  to  wind  and  in  a  case  of  this  kind, 


1231 


FIG.  133  (a)  and  (6). — Connections  for  a  single-phase,  8-pole,  24-slot,  serie* 
connected,  consequent-pole,  induction  motor  shown  at  left.  At  right,  same 
for  series-parallel  connection . 

a  consequent-pole  winding  would  be  used  with  a  distribution 
as  shown  in  Fig.  129.  This  motor  would  be  wound  in  the 
same  manner  as  shown  in  Fig.  125  (a)  to  (g),  but  all  the  coils 
would  be  connected  with  the  same  polarity  as  indicated  in 
Fig.  133. 

Connections  for  Main  and  Starting  Windings. — The  coils 
of  each  winding  are  connected  in  series,  as  shown  in  Fig.  134. 
The  leads  of  the  starting  winding  are  interrupted  by  a  cir- 
cuit opening  device.  Two  circular  stationary  segments  are 
used  in  one  design  of  motor  insulated  from  each  other  and 
from  the  frame  and  are  mounted  side  by  side  on  the  bearing 
housing.  At  the  start  they  are  short-circuited  by  the  rotat- 
ing shoes  which  close  the  circuit  of  the  starting  winding  until 
the  speed  reaches  the  point  at  which  centrifugal  force  throws 
them  off.  This  is  ordinarily  at  about  half  speed.  The  main 


192         ARMATURE  WINDING  AND  MOTOR  REPAIR 


Terminal  Leads==i? 


and  starting  windings  are  usually  connected  in  parallel  outside 
the  machine.  The  direction  of  rotation  is  determined  by  the 
way  the  connections  are  made.  To  reverse  the  direction 
of  rotation,  interchange  either  the  two  main  winding  leads  or 
the  two  starting  leads;  adopting  the  most  convenient  method. 

Testing  Out  Small  Induction 
Motor  Windings. — The  wind- 
ings should  be  tested  for 
grounds  with  1000  volts  for 
one  minute.  Each  winding 
should  also  be  tested  for  short 
and  open  circuits  by  applying 
alternating  current  through  a 
wattmeter.  An  excessive  read- 
ing indicates  a  short  circuit, 
while  no  readings  show  an  open 
circuit.  Polarity  can  be  tested 
by  applying  direct  current  to 
the  windings  and  testing  with 
a  compass.  % Adjacent  poles 
should  attract  opposite  ends 
of  the  needle.  If  the  wind- 

FIG.    134.— Connections *    for   single-   j  are   correct     fog   end   COn: 

phase  motor  windings.  . 

nections   should   be  dipped   in 

a  quick  drying  plastic  varnish,  which  serves  not  only  as  an 
insulating  medium,  but  excludes  all  dust  and  serves  to  stiffen 
the  windings  and  make  them  moisture  proof.  After  dipping, 
the  windings  should  be  thoroughly  dried  in  an  oven. 

Windings  for  Small  Polyphase  Induction  Motors. — The 
method  of  winding  a  small  polyphase  motor  is  similar  to  the 
winding  of  a  single-phase  motor.  The  slots  are  insulated  with 
fish  paper  and  treated  cloth  cells,  and  the  skeins,  which  are 
also  similar,  are  inserted  into  the  slots  as  shown  in  the  winding 
diagrams  of  Figs.  135  and  136.  Each  skein  or  group  of  coils 
overlaps  the  preceding  one  at  the  ends,  as  shown  in  Fig.  137, 
the  ends  of  the  coils  being  separated  from  one  another  by  a 
layer  of  treated  cloth.  To  produce  a  symmetrical  winding,  it 
may  be  necessary  to  remove  half  of  the  first  skein  before  the 
last  one  can  be  put  in  place,  in  order  to  make  them  overlap 


Line  '      , 


REWINDING  ALTERNATING-CURRENT  MACHINES     193 

in  regular  order.  As  the  electrical  characteristics  of  the  ma- 
chine would  not  be  changed  by  so  doing,  the  last  skein  can  be 
allowed  to  overlap  two  skeins,  while  the  first  skein  is  over- 
lapped by  two  others. 


Phase  A 


FIG.   135. — Winding    diagram    and    connections  for  stator  of  small  3-phasc 

motor. 

The  skeins  are  connected  according  to  the  connection  dia- 
gram shown  at  the  right  in  Figs.  135  and  136.  The  connections 
determine  the  phase  and  polarity  of  the  machine.  For  in- 
stance, an  armature  having  twelve  skeins  or  groups  of  coils, 


Phase  A 


Fia.  136. — Winding  diagram  and  connections  for  stator  of  a  small  2-phase 

motor. 

can   be  connected   for  six  poles,   two-phase,  or  eight  poles, 
three-phase. 

No  starting  winding  is  required  for  polyphase  machines,  and 
since  they  have  a  good  starting  torque  there  is  no  need  for  a 

13 


194         ARMATURE  WINDING  AND  MOTOR  REPAIR 


friction  clutch  on  these  machines.  They  should  be  tested 
for  breakdown  between  phases,  and  from  each  phase  to  ground, 
and  for  short  and  open  circuits,  as  described  for  single-phase 
machines. 


FIG.  137. — Stator  of  a  small  3-phase  motor  completely  wound  with  skein  coils. 


II.  WINDING    SMALL    INDUCTION    MOTORS    WITH 
COILS  IN  PARTIALLY  CLOSED  SLOTS 


FORMED 


Like  the  armatures  of  direct-current  machines,  the  stators 
of  small  induction  motors  are  made  with  partially  closed  and 
with  open  slots.  The  coils  used  with  partially  closed  slots 
are  usually  either  of  a  diamond  shape  or  of  a  square  shape 
such  as  shown  in  Fig.  138.  These 
coils  are  wound  on  forms  as  in  the 
case  of  direct-current  coils  using  wire 
having  a  double-cotton  covering.  The 
coils  when  made  up  are  not  insulated 
as  a  whole,  since  the  turns  of  the  coil 
must  be  inserted  in  the  slot  one  at  a 
time.  The  insulation  between  coils 
and  the  core  must  therefore  be  placed 
in  the  slots  before  the  coils  are  in- 
serted. The  character  of  insulation 
required  and  the  methods  of  placing 
the  coils  in  the  stator  of  motors 
having  partially  closed  slots  and  for  those  having  open  slots 
are  outlined  in  what  follows  according  to  recommendations 
by  a  writer  in  the  Electric  Journal,  Vol.  VII,  No.  9. 


FIG.  138.  — At  left,  a 
basket  or  mush  type  coil. 
At  right  a  diamond  type  coil. 


REWINDING  ALTERNATING-CURRENT  MACHINES     195 


Fleh  Paper  Cell 


I- — Fttx*  Strip 


FIG.  139.— Slot  insula- 
tion for  a  small  induction 
motor  having  partially 
closed  slots. 


Insulation  for  Slots. — Clean  the  slots  with  a  blast  of  air  to 
remove  all  burrs  and  rough  edges.  Then  cut  a  fish-paper  cell 
large  enough  to  just  line  the  slot  up  to  the  opening  and  so  as 
to  extend  at  each  end  of  the  slot  about  three-quarters  of  an 
inch.  This  fish-paper  cell  possesses  sufficient  mechanical 
strength  to  protect  the  inner  layer  or  cell  of  insulation  which 
should  be  of  treated  cloth  that  have  high  insulating  strength 
such  as  empire  cloth  or  micanite.  The  inner  cell  should  be 
cut  large  enough  to  extend  beyond  the  protecting  cell  and  the 
ends  of  the  slot  about  one-quarter  of  an 
inch  and  up  through  the  opening  in  the 
slot  about  an  inch.  The  projections  of 
this  cell  serve  as  a  guide  when  inserting 
the  coils  and  protect  the  wires  from 
any  possible  abrasion  when  being  in- 
serted. The  winding  and  the  core 
should  be  insulated  at  the  ends  of  the 
slots  by  a  fullerboard  ring  and  a  fiber 
strip  at  each  end.  This  will  protect 
the  overhanging  ends  of  the  coils  from  coming  into  contact 
with  the  core. 

Basket  Coils — One  Coil  per  Slot. — Basket  coils  are  wound 
to  a  shape  corresponding  only  approximately  to  their  final 
shape,  as  shown  at  the  left,  Fig.  138  It  is  important  only 
that  the  total  length  of  the  loop  be  correctly  proportioned  so 
that  the  ends  may  be  formed  to  the  proper  shape  after  the 
wires  are  in  the  slot.  When  a  suitable  form  is  not  available, 
the  coil  may  be  wound  around  two  pegs  spaced  the  proper  dis- 
tance apart.  Small  pieces  of  tape  should  be  fastened  around 
the  coil  at  convenient  points  so  that  the  wires  may  be 
held  together  while  they  are  being  threaded  into  the  slot. 
Basket  coils  are  largely  used  with  a  one-coil  per  slot  wind- 
ing. This  means  that  each  side  completely  fills  one  slot, 
so  that  for  a  48-slot  core  only  24  coils  are  required.  In 
this  class  of  winding,  one  end  of  the  coil  has  to  drop  below 
the  bottom  of  the  slot  in  order  to  permit  the  adjacent  coil 
to  pass  over  it,  while  the  other  side  of  the  same  coil  remams 
on  a  level  with  or  only  slightly  below  the  top  bore  of  the 
slot. 


196 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


Winding  a  Three-phase  Stator  with  Basket  Coils. — Assume, 
for  example,  that  a  48-slot  core  is  to  be  wound  for  a  four-pole, 
three-phase  machine  with  a  coil  throw  (in  slots)  of  1  and  12. 
Mark  two  slots  to  serve  as  a  guide  in  placing  the  coils  with 
respect  to  their  proper  span  or  throw.  Any  slot  may  be 
considered  slot  1  and  the  other  slot  located  by  counting 
the  throw  in  a  clockwise  direction.  In  this  case  (Fig.  140)  the 
lower  side  of  the  first  coil  will  go  in  slot  12.  This  side  of  the 
coil  must  now  be  laid  wire  by  wire,  inside  the  insulating  cell 

in  slot  12.  The  pro- 
jecting edges  of  this 
cell  can  then  be  cut 
off  close  to  the  lami- 
nations, the  ends 
folded  over  one  an- 
other and  the  slot 

FIG.  140. — Winding   diagram   for  motor   using     closed     by    driving    a 
basket  or  loosely  formed  coils.  tight-fitting    fi b 6 r 

wedge  between  the  outer  cell  and  the  tips  of  the  teeth.  This 
retaining  wedge  should  not  extend  more  than  1/2  incn  beyond  the 
core  on  either  side,  as  it  is  liable  to  curl  up  and  rub  against  the 
rotor.  That  part  of  the  coil  projecting  beyond  the  slot  should 
now  be  taped  with  a  layer  of  treated  tape  and  a  covering  of 
cotton  tape  for  about  half  its  length  on  each  side  of  the  core, 
and  formed  to  drop  below  the  bottom  of  the  slot.  This  is 
known  as  the  bottom  part  of  the  coil,  as  distinguished  from 
the  other  side  which  will  remain  on  a  level  with  the  slots 
known  as  the  top  part.  In  continuing  the  winding,  the  top 
part  should  be  left  out  for  the  present,  of  slots  1,  3,  5,  7  and  9, 
since  in  completing  the  winding  the  coils  in  slots  2,  4,  6,  8 
and  10,  which  drop  below  the  slots,  must  be  in  place  before 
these  top  parts  can  be  inserted.  These  coils  which  have  one 
side  left  out  of  the  slot  until  the  rest  of  the  winding  is  com- 
pleted, are  known  as  throw  coils.  In  the  present  case  they 
are  coils,  1-12,  3-14,  5-16,  7-18,  9-20. 

The  first  coil  that  can  be  wound  into  two  slots  as  a  top  and 
bottom  coil  is  11-22.  The  coil  in  slot  22,  being  a  bottom  coil, 
is  inserted  in  the  same  manner  as  the  coil  in  slot  12  already 
described,  but  is  not  taped  or  shaped  until  its  other  side  is 


REWINDING  ALTERNATING-CURRENT  MACHINES     197 

threaded  into  slot  11.  The  ends  can  then  be  taped  from  iron 
to  iron  with  treated  tape,  in  such  a  way  as  to  overlap  and  seal 
the  projecting  end  of  the  insulating  cell.  This  in  turn  should 
be  covered  with  a  layer  of  cotton  tape.  The  lower  part  of 
the  coil  should  then  be  shaped  with  a  rubber  or  rawhide  mallet 
and  fiber  drift  and  treated  with  an  insulating  compound  before 


FIG.  141. — In  this  illustration  the  armature  winder  is  shown  inserting  the 
turns  of  a  basket  coil  into  partially  closed  slots  of  a  small  induction  motor 
stator  (Westinghuse  Electric  &  Mfg.  Company). 

the  next  coil  is  put  in  place.  The  drift  and  mallet  used  in 
shaping  the  coils  should  have  their  sharp  corners  smoothed 
off,  and  in  no  case  should  the  coils  be  struck  directly  with  an 
iron  tool. 

Fig.  142  is  a  partially  wound  stator,  showing  the  finished 


198 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


shape  of  the  coil  ends.  It  is  particularly  necessary  that  both 
tapings  should  be  applied  as  tightly  as  possible,  as  a  failure 
to  watch  this  point  will  result  in  the  taping  becoming  loose 
and  baggy  when  the  coil  is  shaped.  This  is  especially  true 
if  the  slot  is  very  deep  as  the  process  of  dipping  or  shaping 
the  bottom  coil  below  the  bottom  of  the  slot  has  a  tendency 
to  pull  the  tape  away  from  the  laminations.  To  avoid  this 
tendency  it  is  desirable  to  use  glue  on  those  turns  of  tape 
that  cover  the  projecting  ends  of  the  fish-paper  cell. 


FIG.  142. — A  three-phase  stator  partly  wound  with  basket  coils. 

During  the  operation  of  taping,  the  beginning  and  end  of 
each  coil  should  be  brought  out  in  such  a  manner  as  to  lie  on  the 
side  of  the  winding  farthest  from  the  bore  of  the  stator  and  so 
placed  that  the  beginning  of  one  coil  faces  the  end  of  the 
adjacent  coil  in  a  manner  convenient  for  connecting.  The 
method  of  winding  just  described  should  be  continued  until 
all  the  coils  are  in  place.  The  winding  is  completed  by  placing 
the  top  parts  of  the  throw  coils  in  their  respective  slots. 

Threaded  Diamond  Coil— Two  Coils  Per  Slot. — The 
threaded  type  of  diamond  coil  is  wound  with  insulated  wire, 
either  on  a  form  which  finishes  the  coil  to  shape,  or  on  a  shuttle 
which  winds  the  coil  first  in  the  shape  of  a  loop,  after  which 
the  loop  is  placed  in  a  universal  former  or  " puller."  In 
this  device  (see  page  457,  Chapter  XVII),  the  two  straight 


REWINDING  ALTERNATING-CURRENT  MACHINES     199 

sides  are  clamped  between  suitable  jaws  and  are  pulled 
apart  the  required  distance,  the  coil  assuming  the  shape 
shown  on  the  right  in  Fig.  138.  This  type  of  winding  is  similar 
to  the  basket  winding,  in  so  far  as  the  slot  is  insulated  instead 
of  the  coil.  On  the  other  hand,  the  structure  and  the  shape 
of  the  finished  coils  is  identical  with  that  of  an  open  slot 
insulated  coil.  It  is,  therefore,  not  necessary  to  shape  the 
coil  after  it  has  been  placed  in  the  slot  as  in  the  case  of  the 
basket  winding.  The  slot  insulation  is  practically  identical 
with  that  of  the  basket  winding  with  the  addition  of  a  fiber 
center  strip  to  separate  the  upper  and  lower  coils  in  the  same 
slot.  In  this  case,  however,  the  slots  should  be  laid  out  in 
groups  according  to  a  winding  diagram,  the  number  of  groups 
being  equal  to  the  product  of  the  number  of  poles  by  the  phase 
of  the  motor.  This  grouping  may  be  either  uniform,  alternate 
or  irregular,  according  to  the  design  of  the  winding.  When 
the  grouping  is  uniform,  all  groups  consist  of  an  equal  number 
of  slots,  and  contain  an  equal  number  of  coils.  In  alternate 
grouping,  every  other  group  contains  an  equal  number  of 
slots  and  coils.  In  irregular  grouping  there  is  no  apparent 
uniformity  in  the  number  of  slots  and  coils  per  group. 

Winding  a  Three-phase  Stator  with  Diamond  Coils. — In 
preparing  to  wind  the  machine,  the  slots  forming  the  beginning 
of  each  group  should  be  marked  to  indicate  that  the  coils 
to  be  placed  in  them  must  be  furnished  with  additional  in- 
sulation at  the  ends.  This  is  necessary  as  these  coils  form  a 
boundary  between  phases,  and  consequently  are  subject 
to  phase  potential,  whereas  the  potential  between  any  other 
two  adjacent  coils  is  much  less.  Such  coils  are  called  the 
" phase  coils"  of  the  winding. 

Taking  as  an  example  a  72-slot,  six-pole,  three-phase  motor 
with  a  throw  of  1  and  11.  There  will  be  6  X  3  =  18  groups 
of  four  coils  each,  with  18-taped  "phase  coils"  in  the  winding. 
The  winding  of  the  coils  into  the  slots  can  be  started  by  thread- 
ing the  bottom  of  the  first  coil  into  slot  11,  the  upper  half 
of  the  coil  being  left  up  as  a  throw  coil.  The  bottom  parts 
of  the  next  nine  coils  should  be  inserted  in  rotation,  thus  mak- 
ing ten  coils  left  up  as  throw  coils.  The  succeeding  coil  should 
be  placed  in  two  slots,  11  and  21.  The  rule  to  follow  is, 


200         ARMATURE  WINDING  AND  MOTOR  REPAIR 

that  no  upper  part  of  a  coil  can  be  placed  into  a  slot,  the  lower 
part  of  which  is  empty.  After  each  bottom  coil  has  been 
threaded  into  its  cell,  both  the  insulating  cell  and  the  coil 
should  be  raised  to  the  top  of  the  slot  and  the  projecting 
sides  of  the  cell  cut  off  as  close  to  the  laminations  as  possible. 
The  coil  must  then  be  forced  back  to  the  bottom  of  the  slot. 
The  edges  of  the  insulating  cell  should  be  folded  over  each 
other  and  held  in  place  by  the  fiber  center  strip.  The  latter 
should  be  of  a  width  to  make  a  driving  fit  in  the  slot  so  as  to 
hold  the  lower  coil  and  cell  firmly  in  place,  and  should  be  long 
enough  to  project  out  of  the  slot  beyond  the  straight  part 
of  the  coil. 

Every  fourth  coil,  in  the  present  case,  is  one  of  the  special 
" phase  coils,"  and  should  be  taped  at  the  end  from  iron  to 
iron  with  an  overlapping  layer  of  insulating  tape  and  a  protec- 
tive layer  of  cotton  tape.  The  remaining  coils  may  be  left 
without  any  special  insulation  at  the  ends. 

The  placing  of  the  upper  part  of  the  coil  in  the  slot  requires 
more  care  and  skill,  since  the  remaining  space  in  the  slot  is 
just  large  enough  to  receive  the  coil  with  its  wires  lying  parallel 
to  each  other.  Fig.  139  shows  the  cross-section  of  a  coil 
three  wires  wide  by  four  deep,  the  wire  being  threaded  through 
in  such  a  way  that  wire  1  is  first  passed  through  the  slot, 
then  3  and  finally  2.  This  should  be  continued  with  each 
succeeding  layer  until  the  whole  coil  is  in  place.  The  cell 
can  then  be  folded  in  and  the  slot  closed  with  a  fiber  wedge. 
The  two  coils  must  completely  fill  the  slot.  If  the  insulation 
already  mentioned  is  not  sufficient  to  do  this,  additional 
filling  strips  of  fullerboard,  or  treated  wood,  should  be  packed 
into  the  bottom  or  sides  of  the  slot  as  may  be  required,  until 
the  coil  is  tight  enough  to  prevent  movement  in  the  slot. 
The  upper  and  lower  parts  of  the  winding  outside  of  the  slots 
must  be  carefully  insulated  from  each  other  by  a  strip  of 
treated  duck  cloth.  This  strip  should  be  wide  enough  to 
extend  from  the  fish-paper  cell  to  the  portion  of  the  coil 
farthest  away  from  the  slot,  and  should  be  threaded  between 
the  ends  of  the  coils  as  they  are  inserted  in  the  slots. 


REWINDING  ALTERNA  TING-CURRENT  MACHINES     201 


III.  WINDING  INDUCTION  MOTORS  HAVING  OPEN  SLOTS 

The  main  difference  between  the  coils  used  for  a  partially 
closed-slot  and  an  open-slot  winding,  is  that  the  latter  are  com- 
pletely insulated  before  being  inserted  in  the  slots.  They 
may,  therefore,  be  wire  or  bar  formed,  and  can  readily  be 
insulated  for  any  commercial  voltage.  Consequently,  motors 
of  large  size,  or  for  voltages  exceeding  550  volts  are  nearly 
always  of  the  open-slot  type.  The  insulation  over  the  coils 


FIG.  143. — Stator  winding  of  a  Fairbanks- Morse  induction  motor  showing 
the  type  of  coil  used. 

consists  of  a  cell  of  treated  cloth  or  mica  over  the  straight  part, 
and  an  overlapping  layer  of  cotton  tape  over  the  whole  coil. 
Extra  insulation  for  high  voltages  (see  pages  163  to  172)  is 
made  up  by  extra  thickness  or  extra  turns  of  the  insulating  cell. 
The  phase  coils  receive  an  extra  wrapping  of  treated  cloth  tape 
over  the  diamond  ends  in  addition  to  the  cotton  tape,  which, 
as  a  distinguishing  mark,  may  be  of  a  special  color  on  these 
coils.  After  the  final  wrapping  is  completed,  the  coils  should 


202         ARMATURE  WINDING  AND  MOTOR  REPAIR 

be  dried  in  a  moderate  temperature  in  order  to  expel  all  mois- 
ture, then,  while  still  hot,  they  should  be  dipped  in  an  insula- 
ting compound  and  again  subjected  to  moderate  heat  until  the 
compound  is  thoroughly  dried.  This  compound  serves  to  fill 
up  all  the  pores  in  the  insulating  materials  and  make  the  coils 
dust  and  moisture  proof.  Before  inserting  the  coils  in  the 
slots  the  latter  should  be  lined  with  cells  of  paraffined  fish 
paper  cut  wide  enough  to  project  out  of  the  top  of  the  slot  when 
folded,  and  to  serve  as  a  guide  to  the  coils.  These  winding 
cells  sometimes  called  "slippers"  furnish  mechanical  protec- 
tion only  to  the  coils. 

Winding  a  Two -phase  Stator  Having  Open  Slots. — Assume 
a  stator  designed  for  six  poles,  72  slots,  two-phase,  with  a  coil 
throw  (in  slots)  of  1  and  12.  The  total  number  of  groups  with 
uniform  grouping  is  equal  to  the  product  of  the  number  of 
poles  by  the  phase  of  the  motor,  in  this  case  will  be  12,  with  six 
slots  each.  The  groups  should  be  laid  off  in  a  clockwise 
direction,  the  outside  slots  of  each  group  being  marked  to 
receive  phase  coils,  of  which  there  are  two  per  group.  The 
coils  can  be  inserted  in  regular  order,  beginning  with  the 
bottom  part  of  phase  coil  in  slot  12,  and  inserting  phase  coils 
wherever  indicated.  They  should  be  driven  into  place  by 
means  of  a  fiber  drift  and  mallet.  Paraffin  may  be  used  as  a 
lubricator,  if  necessary,  as  the  coil  should  be  a  good  driving  fit 
in  the  protecting  cell.  The  top  parts  of  the  first  eleven  coils, 
which  are  to  be  the  throw  coils  will  be  inserted  only  temporarily 
until  the  bottom  coils  have  been  inserted  into  these  slots. 
The  remaining  coils  should  be  driven  tightly  into  place,  the 
projecting  edges  of  the  winding  cell  trimmed  off  close  to  the 
core  and  folded  in,  and  the  slot  closed  by  driving  a  fiber  retain- 
ing wedge  into  the  grooves  at  the  top  of  the  slot.  In  some 
machines  this  wedge  covers  the  entire  face  of  the  coil,  while 
in  others  short  wedges  are  driven  in  at  each  side,  leaving  the 
coil  exposed  at  the  middle  of  the  core. 

Testing  the  Windings. — After  all  the  coils  are  in  place  on 
either  of  the  types  described,  the  winding  must  be  carefully 
inspected  for  mechanical  defects.  All  coils  must  clear  the 
bore  of  the  stator  by  at  least  one-sixteenth  of  an  inch,  and  any 
coil  which  obstructs  the  bore  should  be  tapped  down  with  a 


REWINDING  ALTERNATING-CURRENT  MACHINES     203 

mallet  and  fiber  drift  to  give  the  allowed  clearance.  All  cells 
must  be  intact  and  sound,  especially  at  the  bottom  of  the 
slots.  If  the  punchings  have  been  spread  apart  by  driving  in 
the  fiber  wedges,  they  must  be  closed  up  again.  The  punch- 
ings should  also  be  inspected  to  see  that  no  fragments  project 
into  the  bore  of  the  stator  where  the  rotor  will  rub  against 
them. 

The  winding  should  then  be  subjected  to  a  break-down  test 
to  make  sure  that  the  insulation  is  sound  at  those  points  which 
in  service  will  receive  the  most  strain.  These  points  are,  from 
coils  to  iron,  and  from  phase  to  phase.  Connect  all  coils  of 
the  same  phase  together  by  a  piece  of  copper  wire.  Then 
connect  the  terminals  of  a  testingtransformer  between  the  dif- 
ferent phases  and  from  each  phase  to  iron  and  apply  the  re- 
quired voltage  for  a  length  of  time  depending  upon  the  charac- 
teristics of  the  machines.  (See  page  175,  Chapter  VII.)  In 
case  this  test  punctures  the  insulaton  of  any  coil  or  the  insula- 
ting cell  in  any  of  the  slots,  this  insulation  must  be  removed 
and  replaced.  If  the  puncture  occurs  in  a  top  coil,  it  may 
sometimes  be  repaired  by  raising  the  coil  out  of  the  slot, 
removing  the  punctured  insulation,  and  carefully  replacing 
it  with  new  material.  If,  however,  a  bottom  coil  or  cell  is 
punctured,  it  must  be  removed  by  raising  the  tops  of  the  over- 
lapping coils.  The  same  procedure  must, be  followed  if  a 
coil  has  to  be  replaced,  as  in  the  case  of  a  burn-out  due  to  a 
short-circuit.  Such  a  process  amounts  practically  to  retracing 
the  operation  performed  in  the  original  winding  until  the 
injured  coil  is  exposed. 

Inserting  a  New  Coil  in  a  Winding. — In  repair  work  where 
only  one  coil  of  a  basket  winding  is  burned  out  or  damaged, 
and  in  general  if  the  coils  have  been  painted  and  are  stiffened, 
rather  than  to  remove  all  the  throw  coils,  it  is  frequently 
easier  to  thread  in  a  new  coil.  To  do  this  cut  the  damaged 
coil  at  each  side  of  the  core  and  pull  the  wires  out.  New  fish 
paper  and  treated  cloth  cells  must  be  inserted.  In  emergency, 
f  ullerboard  or  rope  cement  paper  may  be  used  in  place  of  fish 
paper.  The  treated  cloth  is  variously  known  as  treated 
cloth,  oiled  linen,  empire  cloth,  etc.  (See  Chapter  VII.) 
In  this  operation  coils  can  be  bent  slightly  to  get  them  out  of 


204 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


the  way.  Double  cotton-covered  wire  of  correct  size  and  suit- 
able length  for  rewinding  must  be  used.  The  length  and  size 
can  be  obtained  from  the  old  coil  by  counting  the  turns  and 
measuring  the  length  of  one.  To  make  the  wire  thread  in 
easily  it  can  be  rubbed  with  paraffine.  If  the  coil  consisted 
of  two  or  more  wires  in  parallel,  that  number  should  be  used 
in  rewinding.  The  number  of  wires  in  parallel  is  the  number 
entering  a  given  coil  from  any  junction  point  of  coil  terminals. 
Great  care  must  be  exercised  during  the  threading  process  to 


FIG.   144. — Method  of  threading  in  a  new  coil  to  replace  a  damaged  one. 

see  that  all  the  wires  lie  parallel  in  the  slots  and  are  free  from 
kinks.  Otherwise  it  will  be  impossible  to  get  the  full  number  of 
turns  into  the  slot.  In  case  a  large  number  of  turns  are  needed 
(more  than  20  per  slot)  two  or  more  lengths  of  wire  may  be 
wound  at  the  same  time  and  connected  in  series  after  winding, 
the  joint  being  made  outside  of  the  slot.  When  the  required 
number  of  turns  have  been  inserted,  the  edges  of  the  cells 
should  be  trimmed  and  folded  in,  the  coil  wedged  in  place  and 
taped  at  each  end.  It  should  then  be  painted  with  an  insula- 
ting paint. 


REWINDING  ALTERNATING-CURRENT  MACHINES    205 

Connecting  the  Coils. — After  the  ground  test,  the  coils 
can  be  connected  into  groups,  by  joining  together  the  begin- 
nings and  ends  of  adjacent  coils  until  the  group  has  but  one 
lead  at  each  end  unconnected,  which  form  the  leads  of  that 


FIG.  145. — The  stator  of  an  inductor  motor  with  coils  in  place  ready  for 
connecting  the  pole-phase-groups  and  making  end  connections  of  such 
groups. 

group.  In  doing  this,  the  wires  should  be  scraped  clean 
and  a  sleeve  connector  of  tinned  copper  slipped  over  them. 
They  can  then  be  soldered  and  taped.  If  suitable  connectors 
can  not  be  obtained  (as  may  occur  in  making  repairs),  the 


206          ARMATURE  WINDING  AND  MOTOR  REPAIR 

stubs  may  be  wrapped  with  fine  bare  copper  wire  and  then 
soldered. 

The  terminals  of  the  various  groups  should  next  be  connected 
into  proper  phase  relations  (see  Chapter  XI)  with  double- 
braided  rubber-covered  wire  or  cable.  The  size  of  wire  and 
the  grade  of  insulation  will  depend  upon  the  current  and  vol- 
tage. Joints  in  cables  or  wire  larger  than  No.  6  should  be 
made  by  wrapping  with  fine  bare  copper  wire.  Great  care 
must  be  taken  while  connecting  and  soldering  joints  to  pro- 
tect the  winding  from  the  molten  metal.  After  soldering, 
all  joints  and  splices  should  be  rubbed  smooth  with  emery 
cloth  and  insulated  with  treated  cloth  tape,  covered  with 
one  or  more  layers  of  cotton  tape,  and  saturated  with  an 
insulating  varnish  or  shellac.  The  connecting  cables  should 
then  be  arranged  over  the  end  of  the  windings  in  such  a  manner 
as  to  occupy  the  least  possible  space  and  yet  keep  them  clear 
from  the  frame  or  end  brackets. 

Points  to  Consider  when  Connecting  Coils. — The  fol- 
lowing points  should  be  carefully  watched  in  order  to  make  a 
good  job  in  connecting  induction  motor  windings. 

1.  All  wires  should,  if  possible,  be  tinned  before  the  connectors  are 
put  on.     If  this  cannot  be  done  they  must  be  thoroughly  cleaned  by 
scraping. 

2.  All  soldering  must  be  thoroughly  done,  making  a  smooth  and 
solid  joint. 

3.  The  wires  of  the  joints  should  lap  if  space  permits. 

4.  No  acid  flux  should  be  used. 

5.  The  wires  or  cables  must  be  arranged  in  such  a  way  as  to  occupy 
the  least  space  and  not  obstruct  ventilation. 

6.  Wires  or  cables  must  be  clamped  or  tied  down  to  avoid  vibration. 

7.  Joints  must  be  carefully  insulated. 

8.  After  connecting,  the  winding  should  be  tested  for  electrical 
balance  by  causing  a  current  to  flow  through  it  and  comparing  the 
voltage  and  current  readings  of  each  phase. 

The  windings  on  the  side  on  which  the  connections  are  made, 
usually  called  the  front  side,  are  as  a  rule  more  rigid  than  those 
on  the  rear  side,  due  to  the  fact  that  the  connections  exert  a 
bracing  effect.  Since  the  windings  in  the  rear  lack  this  brac- 
ing effect,  it  is  usually  necessary  with  diamond  windings  to 


REWINDING  ALTERNATING-CURRENT  MACHINES    207 

supply  a  supporting  ring  of  insulated  steel  to  which  all  the 
coils  are  laced  with  rope  made  of  six-  or  eight-ply  waxed  ends. 
The  inside  diameter  of  this  ring  should  just  be  large  enough 
to  make  a  good  fit  over  the  winding.  If  it  is  much  larger  than 
that  of  the  winding,  the  coils  will  be  under  tension  which 
will  in  time  tend  to  loosen  them  from  the  supporting  ring. 
Care  should  be  exercised  in  lacing  the  coils  to  the  ring,  when 
the  rope  is  led  through  ends  of  the  coils  with  a  steel  needle, 
to  see  that  no  damage  is  done  to  the  insulation. 

Cleats  and  Terminals. — The  wires  that  tap  into  the  winding 
and  extend  to  the  outside  of  the  frame  are  called  the  leads. 
To  avoid  the  possibility  of  these  leads  transmitting  jerks 
and  vibrations  to  the  soldered  joints  of  the  winding,  they 
should  be  fastened  to  the  frame  by  iron  or  porcelain  cleats. 
Such  cleats  should  also  be  used  to  support  the  leads  from  sag- 
ging wherever  they  pass  close  to  the  iron  or  moving  parts. 

Painting. — All  of  the  windings,  and  especially  the  taped 
joints,  should  be  thoroughly  brushed  with  shellac  or  a  finish- 
ing varnish.  This  material  seals  up  any  porous  places  in  the 
insulation,  and  excludes  dirt  and  moisture  (see  page  176). 

IV.  INDUCTION  MOTOR  SECONDARIES 

The  squirrel-cage  secondary  is  the  simplest  mechanically, 
and  at  the  same  time  is  the  most  rugged  and  compact  form 
of  moving  element  to  be  found  in  any  electric  motor.  The 
operating  characteristics  of  a  squirrel-cage  rotor  are  dependent 
on  its  resistance.  A  winding  of  low  resistance  will  have 
good  efficiency  and  small  slip,  but  will  have  poor  starting 
torque  for  a  given  maximum  current.  A  winding  of  high 
resistance,  on  the  other  hand,  will  have  lower  running  effi- 
ciency and  large  slip,  but  will  give  a  high  starting  torque  with 
minimum  current,  and  is  suitable  for  mill  or  crane  work 
where  starting  under  heavy  load  is  frequent  and  operation 
is  for  short  intervals  only.  Where  it  is  necessary,  however, 
to  start  a  heavy  load  with  small  starting  current,  and  operate 
for  long  intervals  with  good  efficiency,  or  wherever  it  is  neces- 
sary to  vary  the  speed  or  operating  characteristics  of  the  motor 
from  time  to  time,  a  wound  secondary  should  be  used.  The 
windings  have  a  low  resistance,  and  are  connected  in  star 


208 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


with  the  open  ends  connected  to  slip  rings.  Adjustable  ex- 
ternal resistance  can  then  be  connected  in  series  when  start- 
ing, after  which  the  rings  can  be  short-circuited. 


FIG.  146. — Squirrel-cage  rotor  for  a  350-hp.,  3-phase,  440-volt,  1465  r.p.m. 
induction  motor  (Crocker- Wheeler  Company). 

Squirrel -cage  Secondaries. — The  core  of  a  squirrel-cage 
rotor  is  built  up  of  laminated  steel  on  a  spider,  and  keyed 
in  place  with  a  feather  key  on  the  spider  and  ring  keys  at  the  ends. 


FIG.  147. — Solid  metal  rotor  "cage"  winding  of  squirrel  cage  induction  motor 
(Fairbanks-Morse  &  Company). 

This  rotor  cage  is  made  by  casting  end  rings  of  copper  or  brass  around  the  ends  of  the 
rotor  bars  which  have  previously  been  treated  so  that  the  metal  of  the  bars  is  melted  and 
fused  together  with  the  metal  of  the  ring  into  a  solid  mass.  This  makes  a  rotor  cage 
which  is  of  one  piece  without  joints.  The  iron  core  of  the  rotor  was  removed  in  order  to 
obtain  this  photograph  of  the  winding  only,  to  show  the  characteristic  structure  from 
which  the  name  "squirrel  cage"  is  derived. 

Ventilators  are  used  on  the  rotors  with  wide  cores.     Rectangular 
copper  bars  are  usually  used,  cut  from  soft  drawn  bar  stock. 


REWINDING  ALTERNATING-CURRENT  MACHINES     209 

The  end  rings  are  made  of  copper,  brass,  or  various  grades 
of  resistance  alloys.  The  resistance  of  the  ring,  and  conse- 
quently the  characteristics  of  the  motor,  depend  upon  both  the 
width  and  thickness  of  the  ring,  as  well  as  its  composition,  and 
may  be  varied  over  a  wide  range  by  changing  these  dimensions. 

Phase-wound  Secondaries. —  With  wound  secondaries 
either  diamond  or  basket  coils  can  be  used.  Partially  closed 
slots  are  usually  used  and  the  slots  are  skewed,  to  prevent 
humming  in  single-phase  designs.  Wire-wound  coils  can  be 
inserted  in  a  manner  as  wire-wound,  threaded-in  coils  for 


FIG.   148. — Large  induction  motor  rotor  partly  wound  with  two-part  strap 
coils  (Crocker-Wheeler  Company), 

direct-current  armatures.  The  slots  should  be  insulated  with 
fish  paper  and  treated  cloth  cells.  The  coil  throw  is  determined 
by  the  number  of  slots  per  pole.  The  coils  in  each  group 
should  be  connected  in  series,  and  the  groups  of  each  phase 
usually  connected  in  series.  The  phases  may  be  connected 
in  star.  Wedges  are  required  to  hold  the  coils  in  the  slots, 
and  the  rotor  should  be  banded  and  balanced  in  the  same 
manner  as  a  direct-current  armature. 

In  rotors  of  large  machines,  the  coils  are  generally  form 
wound,  of  strip  copper,  and  should  be  completely  insulated 
before  insertion  in  the  slots.  For  partly  closed  slots  the  com- 

14 


210         ARMATURE  WINDING  AND  MOTOR  REPAIR 

plete  coil  may  be  composed  of  several  strips,  each  completely 
insulated  with  a  wrapper  of  treated  cloth  and  a  winding  of 
cotton  tape.  In  some  types  of  machines,  designed  for  espe- 
cially heavy  service,  the  insulation  is  composed  of  sheet  mican- 
ite,  wrapped  with  cotton  tape.  A  cell  of  fish  paper  should  be 
inserted  in  the  slot  for  mechanical  protection  and  the  straps 
threaded  in  one  by  one.  The  cell  may  then  be  folded  in  and 
a  wedge  inserted.  Wave  windings  are  used  for  form-wound 
rotors. 

Collector  rings  are  made  of  copper  or  brass.  If  mounted  in- 
side the  bearings,  they  are  usually  provided  with  lugs  which 
are  bolted  through  insulating  washers  to  a  ring  in  a  small 
spider.  If  mounted  outside  the  bearings,  the  leads  are 
brought  out  through  the  hollow  shaft  and  bolted  to  the  rings. 

V.     WINDING  LARGE  ALTERNATING-CURRENT  STATORS 

The  machines  which  fall  under  this  heading  include  large 
induction  motors  used  in  industrial  plants,  engine  and  water- 
wheel  driven  generators,  motor-generator  sets,  synchronous 
motors  and  frequency  changers.  While  in  many  cases  differ- 
ent types  of  windings  are  used,  the  method  of  winding  the 
stators  is  essentially  the  same  in  all  cases.  Particular  refer- 
ence will  be  made,  however,  in  the  winding  details  to  the  usual 
construction  and  requirements  of  induction  motors.  The 
recommendations  for  insulation  of  slots,  make  up  of  coils  and 
their  insertion  are  those  of  a  writer  in  the  Electric  Journal, 
Vol.  VII,  No.  12,  and  represent  good  practice  both  for  closed 
and  open  slot  construction. 

Coils  for  Partially  Closed  Slots. — When  it  is  necessary  to 
replace  the  coils  in  large  machines,  they  should  be  purchased 
from  the  manufacturer  since  it  is  very  difficult  to  properly 
form  and  insulate  a  number  of  large  heavy  coils  with  the  facili- 
ties available  in  an  ordinary  repair  shop  or  at  the  location 
where  the  machine  is  operated,  such  as  a  generating  station 
or  industrial  plant. 

The  partially  closed  slot  requires  a  form  of  coil  which  can 
be  either  threaded-in  through  the  slot  opening  or  inserted 
from  the  end.  As  it  is  ordinarily  impracticable  to  insulate 


REWINDING  ALTERNATING-CURRENT  MACHINES    21 1 

the  slots  for  the  voltages  commonly  used  on  large  machines 
by  the  methods  used  for  threaded-in  coils,  it  is  necessary 
either  to  insulate  each  strand  for  the  full  voltage  or  to  use 
some  form  of  winding  in  which  the  complete  coil  can  be  in- 
sulated from  ground  and  impregnated  and  then  inserted  into 
the  slot  from  the  end. 

The  former  method  may  be  used  where  strap  diamond  coils 
of  a  limited  number  of  turns  are  to  be  used,  and  the  voltages 


FIG.  149. — Large  induction  motor  stator  partly  wound  showing  type  of  coil 
used  (General  Electric  Company). 

are  moderate.  A  standard  application  of  this  method  which 
is  much  used  on  large  induction  motors  consists  of  a  four  coil 
per  slot  winding.  Two  strap  diamond  coils  for  each  slot  are 
completely  insulated  and  impregnated.  The  width  and  depth 
of  the  insulated  coil  are  each  equal  to  half  the  width  and  depth 
of  the  slot.  The  width  of  the  opening  at  the  top  is  one-half 
the  width  of  the  slot,  and  the  winding  process  is  the  same  as 
for  an  open  slot  winding,  except  that  there  are  twice  as  many 
coils  for  the  number  of  slots  as  in  an  ordinary  winding. 

Where  a  number  of  turns  per  slot  are  required  with  a  partly 
closed  slot  a  concentric  shoved  through  coil  can  be  used. 


212 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


This  may  have  as  many  turns  as  desired.  The  coils  may  be 
formed  from  double  cotton  covered  wires,  round  in  the  smaller 
sizes  and  square  in  the  larger.  The  wires  should  be  cut  off  in 
lengths  equal  to  the  total  length  of  the  coil  plus  enough  to 
allow  for  joints,  and  bound  together  in  a  long  straight  bar, 
having  the  correct  cross-section  for  the  coil.  This  bar  is 


FIG.  150. — Section  of  stator  of  water-wheel  driven  generator  showing  chain 

winding. 

then  clamped  by  the  middle  in  a  forming  machine,  and  the 
ends  are  bent  over  suitable  wooden  forms  to  give  the  correct 
shape  to  the  finished  end  of  the  coil.  The  two  free  ends  should 
be  left  straight,  so  that  they  may  be  shoved  through  the  slots. 
Coils  for  Open  Slots. — Although  the  closed  slot  has  some 
advantages  in  the  case  of  induction  motors,  the  form  of  winding 
necessary  for  more  than  four  or  at  most  six  turns  per  slot,  is 


REWINDING  ALTERNATING-CURRENT  MACHINES    213 

complicated  and  expensive  to  wind.  The  coils  for  an  open 
slot  winding  are  easy  and  cheap  to  form,  to  insulate  and  to 
install.  A  diamond  winding  can  be  readily  chorded,  and  thus 
a  standard  frame  and  core  can  be  used  for  different  windings. 
In  addition,  a  coil  which  can  be  completely  insulated  and  im- 
pregnated before  insertion  in  the  slots  is  more  reliable.  Open 


FIG.  151. — Partly  wound  stator  of  a  30-hp.,  3-phase,  60-cycle,    685-r.p.m. 
induction  motor  (Crocker- Wheeler  Company). 

slot  windings  are  used,  therefore,  for  generators  and  synchro- 
nous motors,  and  for  the  large  sizes  of  induction  motors. 

Both  concentric  (also  called  spiral  or  chain)  and  diamond 
windings  can  be  used  with  open  slots  Either  type  of  coil 
(see  page  3,  Chapter  I)  is  formed  at  both  ends,  and  can  be 
completely  insulated  and  impregnated  before  assembling  in 
the  core.  The  concentric  winding  takes  up  less  room  where 


214         ARMATURE  WINDING  AND  MOTOR  REPAIR 

the  throw  of  the  coils  is  great.  All  the  coils  in  a  group  are  of 
different  size  and  shape,  however,  and  the  adjacent  groups 
must  be  of  different  length.  On  account  of  the  number  of 
different  coils  used,  repairs  are  difficult  to  make  and  a  larger 
supply  of  extra  coils  must  be  kept  in  stock.  For  this  reason 
the  diamond  winding  is  quite  generally  used.  A  diamond 
coil  is  of  a  simple  form,  easy  to  build  and  insulate,  and  one 
form  of  coil  is  used  throughout.  Repair  parts  can  thus  be 
reduced  to  a  minimum. 

Lap  and  Wave  Connections. — Either  the  lap  or  wave  form 
of  diamond  winding  may  be  used.  The  end  connections,  by 
which  the  coil  ends  are  connected  into  groups  and  the  groups 
into  phases,  are  very  much  more  simple  with  the  wave  wind- 
ing. This  form  of  winding  is,  therefore,  used  whenever  appli- 
cable. The  voltage  between  adjacent  coil  ends  is  much 
greater,  but  as  the  coil  must  be  insulated  for  full  line  voltage 
under  any  conditions,  this  does  not  make  any  particular  differ- 
ence. Where  more  than  one  turn  per  coil  is  required,  however, 
or  where  a  series-parallel  combination  is  necessary,  the  wave 
winding  is  more  cumbersome  than  the  lap  winding.  It  is 
accordingly  used  only  with  a  one  turn  per  slot  coil,  where  all 
the  groups  in  a  phase  are  connected  in  series.  The  lap  winding 
is  thus  much  more  common  as  most  high  voltage  machines 
require  more  than  one  turn  per  slot.  The  coils  may  be  wound 
from  round  or  square  wire  or  copper  strap,  depending  on  the 
number  of  turns  and  the  size  of  the  conductor. 

Insulation  of  Coils. — The  insulation  to  ground  is  practically 
the  same  for  a  given  voltage  for  all  types  of  coils.  The  wire 
in  wire-wound  coils  should  be  cotton  covered,  and  no  other 
insulation  is  needed  between  turns.  When  the  coils  are  made 
up  of  two  or  more  layers  of  conductors,  however,  the  layers 
should  be  separated  by  drilling,  cotton  tape  or  treated  paper, 
according  to  the  type  of  coil  used.  Strap  conductors  may  be 
insulated  between  turns  with  overlapping  cotton  or  mica 
tape.  In  most  cases,  however,  the  tape  is  used  over  the  ends 
of  the  coils  only,  the  straight  parts  being  insulated  by  inter- 
weaving an  insulating  cell  of  cement  paper  and  mica  between 
them.  Where  several  straps  are  connected  in  parallel  for 
greater  conductivity,  they  are  ordinarily  enameled,  cotton 


REWINDING  ALTERNATING-CURRENT  MACHINES    215 

covered  or  taped  to  prevent  eddy  current  loss.  The  several 
turns,  whether  connected  in  parallel  or  series,  may  be  bound 
together  with  non-overlapping  cotton  tape  and  impregnated 
before  being  insulated  from  ground.  While  drying,  the  straight 
parts  should  be  clamped  in  a  press  so  that  they  will  dry  per- 
fectly straight  and  without  any  interstices  between  the  turns. 
The  hardened  impregnating  gums  then  bind  the  dried  coil 
into  a  compact  unit. 

The  insulation  to  ground  may  consist  of  treated  taping  over 
the  whole  coil,  with  a  protective  covering  of  cotton  tape.  This 
material  is  widely  used  on  the  smaller  machines,  and  for  com- 
paratively low  voltages.  For  larger  machines,  however,  and 
for  high  voltages,  the  customary  insulation  consists  of  a  wrap- 
per of  cement  paper  and  mica  on  the  straight  parts  and  treated 
tape  or  mica  tape  on  the  ends,  with  a  protective  layer  of  un- 
treated cotton  tape  or  mica  tape  on  the  whole  coil,  overlapping 
on  the  ends  and  non-overlapping  on  the  straight  parts.  The 
entire  coil  may  then  be  dipped  twice  in  an  insulating  varnish, 
and  dried  thoroughly  in  an  oven  after  each  dipping.  The 
requisite  insulation  for  the  high  voltage  machines  can  be  se- 
cured by  extra  turns  of  cement  paper  and  mica  wrapper. 
Not  over  three  and  one-half  turns  is  ordinarily  used,  however, 
on  account  of  the  difficulty  of  properly  impregnating  such  a 
coil.  Where  this  does  not  give  sufficient  insulation,  two  or 
more  separate  wrappers  may  be  used,  and  the  coil  dipped 
twice  in  varnish  and  dried  after  the  application  of  each 
wrapper  (see  pages  163  to  172). 

Inserting  Shoved  Through  Concentric  Coils. — Windings 
using  coils  of  this  type  may  be  divided  into  two  classes,  de- 
pending on  whether  the  ends  are  bent  down  at  one  end  or  bent 
down  at  both  ends.  The  winding  processes  are  practically 
similar  for  both.  Coils  bent  down  at  one  end  are  used  on 
both  two-phase  and  three-phase  machines,  the  two  windings 
being  practically  identical  with  the  exception  of  the  end  con- 
nectors. In  a  two-phase  machine  the  alternate  groups  are  of 
the  same  phase,  that  is,  all  the  coils  in  each  bank  of  coil  ends 
belong  to  the  same  phase. 

In  a  three-phase  machine,  with  a  winding  of  this  type,  every 
third  group  belongs  to  the  same  phase,  and  the  groups  of  each 


216          ARMATURE  WINDING  AND  MOTOR  REPAIR 

phase  alternate  from  one  bank  to  the  other.  It  should  be 
noted  in  this  case  that  the  groups  of  the  same  phase  do  not  lie 
adjacent  to  one  another,  but  are  separated  by  a  distance 
equal  to  the  pole  pitch  and  that  there  are  only  three  groups  of 
coils  per  pair  of  poles.  Where  the  groups  of  each  phase  have 
one  side  adjacent  to  another  group  of  the  same  phase,  under 
the  same  pole,  with  six  groups  of  coils  per  pair  of  poles,  it  is 
necessary  to  have  three  banks,  to  allow  the  end  connections  to 
cross  one  another.  In  this  case  the  coils  of  two  of  the  banks 
must  be  bent  down  on  both  ends.  The  ends  of  the  third 
bank  are,  therefore,  bent  down  also,  to  secure  uniformity  of 
the  windings. 

Considerable  care  is  required  in  inserting  coils  of  the  shoved- 
through  type  into  the  slots.  The  slots  must  be  first  cleaned 
from  all  foreign  matter.  The  coils  should  then  be  rubbed 
with  paramne,  and  the  slots  lined  with  paraffined  fish  paper, 
cut  and  bent  to  an  exact  fit.  The  coil  is  thus  made  to  slip 
easily  into  position,  and  at  the  same  time  is  protected  from 
damage  by  sharp  edges  of  the  iron.  A  tight  driving  fit  is 
absolutely  essential  with  this  type  of  winding.  If,  on  trial, 
the  fit  is  too  loose,  strips  of  treated  fullerboard  or  wood  can 
be  placed  in  the  slot,  .or  taped  to  the  coils. 

The  smallest  coil  in  each  group  should  be  placed  in  the 
slots  first,  and  worked  into  them  both  sides  at  once,  until 
the  formed  end  comes  within  a  short  distance  of  the  iron. 
Wooden  distance  blocks  of  appropriate  dimensions  can  be 
used  to  secure  uniformity.  The  other  coils  may  then  be 
inserted  in  order.  All  the  coils  of  one  bank  should  be  inserted 
before  the  other  bank  is  started.  Where  a  two-bank  winding 
is  used,  and  the  coils  are  bent  down  at  one  end  only,  it  is 
necessary  to  insert  the  coils  of  the  different  banks  from  op- 
posite sides  of  the  core.  By  this  method  the  winder  forms 
only  the  straight  end  of  the  coils,  and  his  work  is  very  much 
simplified.  When  a  three-bank  winding  is  used,  however, 
the  winder  must  bend  down  the  ends  of  all  the  coils  as  he 
connects  them.  In  this  case  it  is  easier  to  insert  all  the  coils 
from  one  end.  After  all  the  coils  are  in  place  a  fiber  retaining 
wedge  should  be  driven  in  over  the  top  of  each  coil  to  hold 
it  tightly  in  place,  the  fit  being  as  close  as  possible  without 


REWINDING  ALTERNATING-CURRENT  MACHINES     217 


Second  Layer 


Lead' 


—  Connections  when  concentric  coils 
are  used. 


damaging  the  coil  or  spreading  the  lamination  at  the  ventilat- 
ing slots. 

In  forming  the  ends  of  the  coils  of  both  straight  and  bent 
down  type,  wooden  blocks  may  be  used  over  which  to  bend  and 
shape  the  conductors.  The  inner  layer  should  be  connected 
first,  the  scheme  of  connections  shown  in  Fig.  152  giving  the 
best  results.  The  connector  consists  of  a  copper  sleeve, 
which  is  soldered  over  the  ends,  the  joints  being  staggered, 
so  that  the  coil  ends 
will  not  be  unnecessar- 
ily  bulky.  While  making 
the  joint  it  is  advisable 
to  protect  adjacent  con- 
ductors from  the  heat 

and    Solder  bv  placing  a 
\ 

layer  of  cloth  or  mica 
between  them.  The  finished  joint  should  be  smoothed  off 
with  a  file  or  with  emery  cloth,  and  then  insulated  with 
treated  cloth  and  friction  tape. 

Each  coil,  during  assembly  should  be  bent  to  correspond 
with  the  curve  of  the  stator,  so  that  no  part  of  the  finished 
coil  will  extend  above  the  bore.  A  piece  of  insulating  material, 
usually  treated  fullerboard,  should  be  placed  between  the 
respective  layers,  which  are  then  bound  together  with  treated 
cloth  and  cotton  tape.  This  serves  both  as  insulation  from 
ground  and  as  mechanical  protection  for  the  coils. 

In  the  process  of  connecting,  care  must  be  exercised  that  the 
ends  of  the  same  conductor  are  not  joined  together,  thus  produc- 
ing a  short-circuited  turn.  This  can  be  prevented  by  testing 
out  with  a  test  lamp.  As  a  precautionary  measure,  however, 
after  the  coils  have  been  all  connected,  each  one  should  be 
tested  out  with  a  testing  transformer.  This  device  (see  page 
126,  Chapter  V  for  construction)  should  be  placed  over  one 
side  of  a  coil  and  a  thin  piece  of  sheet  steel  over  the  other.  If 
there  is  a  closed  circuit  any  place  in  the  coil,  a  heavy  current 
will  flow,  and  the  steel  feeler  will  be  strongly  attracted  to 
the  iron  of  the  core. 

Bar  and  Connector  Winding.  —  This  type  of  winding  consists 
of  solid  copper  bars  which  are  completely  insulated  and  shoved 


218 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


through  the  slots.  One  or  two  bars  may  be  used  per  slot. 
Within  an  inch  or  so  of  the  end,  the  bar  is  uninsulated  and  the 
bare  ends  are  tinned.  End  connections  of  diamond  or  in- 
volute form,  insulated  and  provided  with  tinned  ends,  are 
used  to  complete  the  coil.  In  order  to  facilitate  the  operation 
of  soldering,  the  ends  of  the  connectors  are  slotted  or  drilled, 


FIG.  153.- — Section  of  the  stator  of  a  water-wheel  driven  generator  showing 
two-part  bar  coils,  horn  fiber  slot  insulation,  wood  wedges  and  ventilating 
spaces  in  the  core. 

for  without  these  openings  it  is  difficult  to  force  the  solder 
into  the  center  of  the  joint. 

In  the  case  of  a  two-bar  per  slot  winding,  it  Is  necessary 
that  the  bar  next  to  the  rotor  be  shorter  so  that  the  joints 
at  both  ends  of  the  connectors  will  be  accessible  to  a  soldering 
iron.  In  this  case,  sufficient  filling  must  be  placed  between 
the  two  bars  in  the  slot  to  allow  the  end  connector  to  slip 
over  the  top  bar  with  sufficient  clearance  for  taping.  In  case 


REWINDING  ALTERNATING-CURRENT  MACHINES  '  219 

there  is  not  sufficient  room  in  the  slot  to  allow  of  this  filling, 
the  top  bars  are  cut  away.  The  connector  is  shaped,  as 
shown  in  Fig.  154,  so  as  to  allow  it  to  slip  between  the  lower 
bars  and  over  the  ends  of  the  upper  ones.  All  connectors 
must  be  soldered  in  place  and  the  joints  taped.  With  this  type 
of  winding  repairs  are  very  easily  made,  as  any  bar  can  be 
removed  without  disturbing  the  bars  in  any  other  slot,  It 
is  not  adaptable  to  high  volt- 
ages, however,  on  account  of 
the  limited  number  of  possible 
turns.  Furthermore,  the  end 
connections  are  very  difficult  to 
brace  adequately. 

Diamond  Coils.  —  On  large 
machines  a  split  frame  con-  FlG' 
struction  is  necessary.  After 
the  machine  is  completed  and  tested,  the  windings  are  re- 
moved by  the  maker  at  two  points,  in  order  to  allow  the 
frame  to  be  disconnected  for  shipping.  Stfme  form  of  wind- 
ing must,  therefore,  be  used  that  can  readily  be  disconnected 
and  reconnected  on  arrival  at  its  destination.  This  condition 
is  most  readily  met  by  the  open  slot  diamond  coil.  The 
diamond  winding  is  by  far  the  easiest  to  put  in  place,  to 
remove  when  necessary  without  damage  to  the  coil,  to  con- 
nect into  groups  and  phases,  to  brace  at  the  ends,  etc.  It  is, 
therefore,  much  used  on  large  or  high  voltage  machines. 

The  assembly  of  wave  and  lap  windings  with  these  coils  is 
practically  the  same,  and  the  processes  on  the  larger  machines 
are  similar  to  those  employed  on  the  smaller  ones.  The  slots 
should  be  cleaned  and  lined  with  paper  protective  cells. 
The  coils  may  then  be  inserted  one  after  the  other  in  order, 
no  attention  being  paid  to  grouping,  as  the  coils  are  all  alike. 
On  a  large  machine,  the  careful  wedging  of  the  coils  in  the 
slots  is  especially  necessary,  as  the  mechanical  stresses  that 
are  brought  to  bear  on  account  of  the  heavy  currents  in  case 
of  short-circuit,  are  very  large.  Hence  the  coil  must  fit  very 
tightly  in  its  place.  If  necessary,  strips  of  treated  cement 
paper,  fullerboard  or  wood  should  be  placed  in  the  sides  and 
bottoms  of  the  slots  to  ensure  a  tight  fit. 


220 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


When  inserting  the  coils,  the  first  half  of  each  coil  should 
be  driven  tightly  into  the  bottom  of  the  slot  with  a  fiber 
drift  and  mallet.  The  coil  which  goes  over  it  must  also 
be  driven  snugly  into  place.  The  fish-paper  cell  can  then  be 
cut  off  even  with  the  top  of  the  slot  and  folded  over  the  coil. 
In  order  that  this  cell  may  not  be  torn  by  the  wedge,  a  strip 
of  treated  fullerboard,  the  full  width  of  the  coil,  may  be  laid 


FIG.    155. — Stator    of    a    3-phase    Fairbanks-Morse  belted  type  alternator 
wound  with  the  coils  shown  in  Fig.  117. 

into  the  slot,  so  that  the  wedge  will  slide  over  it  readily,  and 
at  the  same  time  compress  the  coil  tightly  into  the  slots. 
The  wedges  may,  for  convenience,  be  divided  into  sections 
6  to  8  inches  long  and  driven  into  position  by  means  of  a 
standard  wedge  driver  or  a  blunt  chisel.  If  the  wedge  is 
a  very  tight  fit,  the  coil  should  be  driven  down  with  a  drift 
just  ahead  of  the  wedge. 


REWINDING  ALTERNATING-CURRENT  MACHINES     221 

A  four-coil  per  slot  diamond  winding  used  with  partly 
closed  slot  cores,  is  assembled  the  same  as  any  other  diamond 
winding,  with  the  exception  that  two  coils  lie  side  by  side  in 
both  the  bottom  and  top  of  the  slot.  The  shape  of  the  slot 
is  such  that  no  difficulty  is  encountered  in  inserting  the  second 
coil.  This  is  essentially  an  induction  motor  winding,  being 
used  quite  generally  on  both  stator  and  rotor  of  large  machines. 

Double  Windings. — On  large  induction  motors,  two  speeds 
are  sometimes  secured  by  the  use  of  two  separate  windings  in 
the  same  slots,  connected  for  a  different  number  of  poles. 
In  this  case  the  windings  may  be  so  designed  that  the  coils 
for  one  speed  lie  inside  the  coils  for  the  second  speed.  The 
two  windings  can  be  assembled  at  the  same  time,  each  pair 
of  coils  being  treated  as  if  it  were  a  single  coil  of  a  one  speed 
winding.  Each  slot  thus  contains  four  coils  one  above  the 
other.  In  such  cases  it  is  quite  common  to  have  the  two  wind- 
ings of  widely  different  capacities,  depending,  of  course,  on 
the  load  to  be  carried  at  the  separate  speeds. 

Testing  Windings  of  Large  Machines. — After  the  coils 
of  the  winding  are  all  in  place,  their  free  terminals  should  be 
temporarily  connected  together,  and  a  test  made  for  break 
down  to  ground,  with  the  standard  voltage  for  the  size  and 
type  of  machine  (see  page  175,  Chapter  VII).  After  the  coils 
have  been  connected  into  groups,  and  the  groups  connected 
into  phases  they  should  be  tested  for  break  down  between 
phases,  and  for  short-circuits  between  turns. 

The  break-down  test  consists  of  the  application  of  the  stand- 
ard break-down  voltage  from  the  conductor  to  ground  and 
from  phase  to  phase.  The  test  for  short-circuit  consists  in 
placing  an  alternating-current  magnet  of  the  type  shown  in 
Fig.  101,  page  126  over  one  side  of  the  coil  or  group,  and  hold- 
ing a  light  piece  of  steel  over  the  other  side  of  coil.  If  current 
flows  in  the  coil,  the  piece  of  steel  will  be  attracted.  As  no 
current  can  flow  under  normal  conditions,  this  test  is  a  sure 
indication  of  short-circuits.  It  cannot  be  applied  where  the 
groups  are  connected  in  parallel  but  must  be  made,  in  this 
case,  before  they  are  connected. 

Connecting  the  Coils. — The  number  of  groups  in  any  ma- 
chine may  be  either  one  or  two  per  phase  per  pair  of  poles, 


222         ARMATURE  WINDING  AND  MOTOR  REPAIR 

depending  on  the  arrangement  of  the  coils  and  groups.  The 
coils  of  each  group  are  connected  in  series  but  the  groups  in 
a  phase  may  be  connected  in  series,  parallel  or  series-parallel, 
although  for  voltages  of  2200  or  higher  they  are  nearly  al- 
ways connected  in  series.  The  phases  on  a  two-phase  machine 
are  never  connected  together  inside  the  machine. 

Stator  windings  of  alternating-current  machines  may  be 
connected  either  star  or  delta,  the  choice  depending  upon  many 
conditions.  For  machines  of  2200  volts  and  over  it  is  ad- 
visable to  connect  the  winding  in  star  because  the  voltage 
from  any  one  terminal  to  ground  is  only  (2200  -T-  1.73)  of  the 
voltage  between  terminals  and  therefore  the  winding  has  a  less 
chance  to  break  down  to  ground.  However,  this  is  not 
a  strict  rule  for  sometimes  it  is  more  convenient  to  have  the 
stator  delta  connected  for  the  reason  that  a  smaller  size  of 
wire  can  be  used  and  a  larger  number  of  turns. 

On  a  concentric  winding  the  groups  are  readily  distinguish- 
able. In  a  diamond  or  involute  winding,  the  number  of  coils 
per  group  must  be  counted  off,  and  the  groups  temporarily 
connected  by  bending  together  the  leads  from  a  wire  coil  or 
by  slipping  a  copper  connector  over  the  stubs  from  a  strap 
coil.  They  should  be  permanently  connected  by  soldering 
the  joints  so  made.  On  most  machines,  the  connectors  be- 
tween groups  consist  of  insulated  copper  strap,  with  suitable 
openings  at  each  end  to  slip  over  the  group  leads. 
All  joints  after  being  carefully  soldered,  should  be  smoothed 
up  with  emery  cloth,  so  that  no  sharp-pointed  edges  are  left 
to  damage  the  insulation. 

The  connectors  between  coils,  ordinarily  called  stubs,  should 
be  insulated  with  treated  cloth  tape,  and  a  protective  cover- 
ing of  untreated  cotton  tape.  Large  stubs  are  sometimes 
covered  with  drilling  caps,  sewed  to  shape,  and  painted  with 
insulating  varnish  after  they  have  been  fastened  in  place. 
Treated  tape  should  be  wound  over  the  joints  in  the  insulation. 
The  thickness  of  the  taping  and  other  insulation,  depends  on 
the  voltage  of  the  machine.  The  connectors  between  groups 
are  ordinarily  insulated  and  impregnated  before  they  are  put 
on  the  machine.  The  joints  between  the  connectors  and  the 
group  leads  should  be  insulated  in  the  same  way  as  the  stubs. 


REWINDING  ALTERNATING-CURRENT  MACHINES    223 

Where  double  windings  are  used  they  are  usually  arranged 
for  the  connections  to  be  made  on  opposite  sides  of  the 
machine.  Each  winding  may  be  connected  as  if  it  were 
entirely  independent. 

Bracing  Needed  for  Heavy  Windings. — The  stresses  occur- 
ring in  the  end  connections  of  a  large  machine,  due  to  magnetic 
reactions  between  current  carrying  conductors,  are  quite 
large.  In  addition,  these  effects  are  greatly  magnified  in 
case  of  a  short-circuit  on  the  generator.  Some  method  of 
bracing  these  end  connections  is  therefore  necessary.  This 
ordinarily  takes  the  form  of  an  insulated  steel  ring,  to  which  the 
coils  are  tied  with  heavy  twine.  On  a  concentric  winding 
a  separate  ring  is  used  for  each  bank  of  end  connections.  On 
a  diamond  winding  one  ring  at  each  end  is  sufficient.  On  a 
large  machine,  however,  it  is  usually  necessary  to  brace  this 
ring  by  additional  steel  supports  bolted  to  the  frame. 

VI.  WINDING  THE  STATOR  OF  ALTERNATING-CURRENT 
TURBO-GENERATORS 

Although  the  alternating-current  turbo-generator  falls  in 
the  previous  group  of  large  machines,  on  account  of  its  winding 
being  more  or  less  special,  it  will  be  dealt  with  separately. 
There  are  special  features  of  windings  for  this  class  of  machines, 
owing  to  the  high  speed  at  which  they  are  operated  and  the 
forced  requirement  that  the  inside  diameter  must  be  small  on 
account  of  the  centrifugal  strains  of  the  revolving  field  rotor. 
The  generator  core  is  therefore  long  requiring  large  coils  fre- 
quently more  than  10  feet  long  with  a  span  from  30  to  40 
inches  and  weighing  as  much  as  100  pounds.  One  of  the  best 
discussions  of  the  methods  for  winding  an  alternating-current 
turbo-generator  has  appeared  in  the  Electric  Journal,  Vol. 
VIII,  No  3.  The  winding  details  that  follow  have  been  taken 
from  that  source. 

Coils  for  A.-C.  Turbo-generators. — These  machines  have 
open  slots  and  usually  employ  a  diamond  coil  built  up  in  two 
pieces  for  one  or  several  turns  per  slot.  The  two-piece  coil  is 
used  where  the  throw  of  the  coils  is  very  great,  as  in  large  size 
bipolar  machines  for  25  cycles.  The  one-piece  coil  is  used 


224         ARMATURE  WINDING  AND  MOTOR  REPAIR 

on  smaller  machines,  and  in  fact,  wherever  such  a  coil  is  not  too 
cumbersome  to  handle,  provided  the  complete  coil  can  be 
passed  through  the  bore.  The  coils  of  either  type  may  be 
formed  from  cotton-covered  strap  or  cotton-covered  wire. 
Where  a  conductor  of  large  cross-section  is  required,  it  is  ordi- 


FIG.  156. — End  of  a  large  turbo-generator    slowing  method  of  bracing  end 
connections  to  resist  short-circuit  stresses. 

narily  built  up  of  a  number  of  square  copper  wires  in  parallel,  to 
facilitate  bending  and  to  obtain  proper  lamination.  The 
individual  strands  are  usually  cotton  covered.  Most  turbo- 
generators require  more  than  one  turn  per  slot  in  order  to 
obtain  the  desired  voltage.  These  several  turns  are  bound 


REWINDING  ALTERNATING-CURRENT  MACHINES    225 

together  in  a  mechanical  unit,  insulated  from  each  other  and 
insulated  as  a  group  from  the  frame  of  the  machine. 

The  coils  may  have  their  end  connections  bent  down  at  any 
angle  from  zero  to  90  degrees.  The  involute  coils,  of  course, 
lie  flat  against  the  face  of  the  frame.  Diamond  coils  are  al- 
ways bent  down  at  the  ends  from  30  to  60  degrees,  both  to 
provide  ample  clearance  and  ready  access  to  the  rotor,  and  in 
order  that  they  may  be  more  suitably  braced,  as  shown  in 
Fig.  156.  Especial  care  should  be  taken  to  so  shape  the  coil 
ends  that  cool  air  can  circulate  freely  through  them.  Coilp 
of  several  turns  sometimes  have  the  individual  turns  insulated 
separately  after  they  leave  the  slots  to  give  greater  cooling 
surface  to  the  ends. 

Forming  the  Coils. — Except  for  conductors  of  large  capacity, 
the  conductors  are  formed  from  a  single  copper  strap.  The 
process  of  forming  the  coil  is  the  same  whether  each  conductor 
consists  of  one  bar  or  several  bars  or  straps  in  parallel.  Each 
group  of  wires  or  straps  which  make  up  a  conductor  in  the 
latter  case  may  be  formed  as  a  single  strap  and  then  bound 
together  with  tape  and  treated  as  a  unit.  For  a  one- conductor 
coil,  designated  as  an  open  coil,  a  strap  of  suitable  length  may 
be  bent  at  the  middle  around  a  pin,  forming  a  U  with  the  sides 
separated  very  slightly.  This  loop  should  then  be  mounted 
directly  in  a  mould.  A  steel  pin  through  the  bend  of  the  U 
will  serve  to  hold  the  point  of  the  diamond  vertical  while  the 
sides  are  bent  to  conform  to  the  shape  of  the  mould.  Coils 
of  several  conductors  are  sometimes  built  in  this  same  way,  by 
forming  the  individual  conductors  in  a  group  with  a  copper 
strip  of  the  same  thickness  as  the  insulation  inserted  between 
them.  The  conductors  should  be  separated  and  insulated 
from  one  another,  and  then  insulated  from  ground  the  same 
as  a  one-conductor  coil.  The  several  conductors  can  be  con- 
nected in  series  when  placed  in  the  machine. 

A  coil  of  more  than  one  conductor  formed  from  a  single 
strap  is  known  as  a  closed  coil  and  requires  several  operations. 
The  strap  should  be  first  wound  around  pins  set  in  a  flat 
table,  to  the  required  number  of  turns,  and  then  given  its 
final  shape  by  forming  over  a  mould.  Throughout  the  entire 
operation  the  conductors  must  be  kept  apart  by  straps  of 


226         ARMATURE  WINDING  AND  MOTOR  REPAIR 

metal  to  the  same  distance  that  they  will  be  separated  when 
insulated  and  placed  in  the  machine.  Two-piece  coils  can 
be  formed  from  lengths  of  straight  copper  strap.  The  ends 
should  be  bent  at  a  suitable  angle  to  the  straight  part  of  the 
coil  around  two  pins  spaced  a  distance  equal  to  the  length  of 
the  straight  part.  The  conductor  may  then  be  clamped  in  a 
mould  and  the  ends  bent  into  shape.  The  several  conduc- 
tors in  a  coil  can  be  formed  in  the  same  mould,  and  separated 
by  strips  of  copper  strap. 

Insulation  for  Turbogenerator  Coils. — Since  a  turbogen- 
erator may  at  times  be  subjected  to  very  heavy  overloads, 
the  insulation  should  be  as  nearly  heat  proof  as  possible. 
For  this  reason,  mica  is  much  used.  Where  several  con- 
ductors per  coil  are  required,  the  insulation  between  conduc- 
tors usually  consists  of  flexible  mica  tape  and  mica  cells.  The 
individual  conductors  may  then  be  suitably  bound  together. 
All  coils  should  be  impregnated  with  an  insulating  compound 
before  the  insulation  from  ground  is  supplied.  The  straight 
parts  may  be  clamped  tightly  in  metal  clamps  and  impreg- 
nated, or  may  be  impregnated  without  the  clamps  and  placed 
in  clamps  to  dry.  Either  method  results  in  a  compact,  uni- 
form construction  so  that  the  coil  may  be  fitted  tightly  into 
the  slot  with  minimum  risk  of  damaging  the  insulation. 

The  insulation  from  ground  usually  consists  of  wrappers  of 
paper  and  mica  over  the  straight  part.  The  coil  ends  should 
be  insulated  by  layers  of  treated  tape,  with  a  covering  of 
untreated  tape,  and  the  whole  coil  treated  with  insulating 
varnish.  As  in  the  case  of  engine  type  generators,  if  extra 
insulation  is  required  additional  wrappers  of  the  paper  and 
mica  are  frequently  used,  the  coil  being  dipped  and  dried 
after  each  wrapper  is  applied.  This  makes  the  building  of  a 
high-voltage  coil  a  long  process,  two  weeks  or  more  frequently 
being  required  for  the  completion  of  a  single  coil. 

Testing  Turbogenerator  Windings. — Open  coils  may  be 
tested  for  short-circuits  by  applying  a  suitable  voltage  between 
conductors.  No  short-circuit  tests  are  usually  made  on  the 
individual  closed  coils,  but  an  over-voltage  test  should  be 
applied  after  they  are  on  the  machine.  All  coils  should  also 
be  tested  for  insulation  by  wrapping  the  outside  with  tin 


REWINDING  ALTERNA  TING-CURRENT  MACHINES     227 

foil,  and  applying  from  two  to  two  and  a  half  times  normal 
operating  voltage  across  the  insulation. 

Inserting  the  Coils  in  a  Turbogenerator. — Except  for  the 
great  weight  and  large  size  of  the  coils  which  makes  them  more 
difficult  to  handle,  the  winding  of  a  turbogenerator  is  in  all 
essentials  similar  to  the  winding  of  an  engine-type  generator. 
The  straight  part  of  the  coil  should  be  waxed  and  laid  over 
the  slot  opening,  inside  a  paper  cell  with  which  the  latter  is 


FIG.  157. — Turbo-generator    stator    partly  wound    using    one-piece    coils. 
The  operator  is  binding  the  coils  in  position. 

lined.  In  order  to  avoid  bending  the  coil,  a  wooden  drift 
as  long  as  the  straight  part  may  be  used  to  drive  it  into  the 
bottom  of  the  slot.  The  bottom  halves  of  two-part  coils 
should  be  inserted  first  in  all  slots  in  the  span  of  one  coil, 
the  top  halves  being  inserted  later.  One-piece  co  Is  should  be 
inserted  in  regular  order.  After  each  top  coil  has  been  fitted 
into  place,  the  cell  which  lines  the  slot  may  be  folded  over, 


228         ARMATURE  WINDING  AND  MOTOR  REPAIR 

and  retaining  wedges  in  short  sections  driven  in  place  over  the 
full  length  of  the  coil.  Enough  strips  of  extra  material  must 
be  used  in  the  slot  to  make  the  wedges  fit  tightly  over  the  coil. 
The  wedges  may  be  driven  in  place  by  a  tool  of  special  shape, 
operated  by  a  compressed-air  hammer. 

In  order  to  get  the  bottom  half  of  the  last  few  one-piece 
coils  into  the  slot,  it  is  necessary  to  lift  the  top  half  of  the 
coils  which  go  in  the  same  slots.  With  a  full  pitch  winding 
this  would  mean  one-half  of  the  coils  on  a  two-pole  machine, 
and  one-fourth  the  coils  on  a  four-pole  machine.  With  a 
"chorded"  or  fractional  pitch  winding,  however,  the  number 
of  throw  coils  is  less.  The  throw  of  the  coils  can  be  readily 
seen  in  Fig.  157  which  shows  a  generator  partly  wound  with 
one-piece  coils. 

In  some  cases,  the  improved  space  factors  that  can  be  se- 
cured makes  it  worth  while  to  use  a  one-coil-per-slot  winding. 
This  winding  can  be  inserted  in  the  same  way  as  the  two-coil- 
per-slot  winding,  except  that  alternate  slots  contain,  respec- 
tivelv,  front  and  rear  ends  of  coils. 

Bracing  for  Windings. — The  instantaneous  current  which 
flows  in  case  of  a  short-circuit  to  a  turbo-generator  op,  in  fact, 
any  other  generator,  is  very  large  and  the  magnetic  stresses, 
which  vary  as  the  square  of  the  current  and  the  sp^n  of  the 
end  windings  are  consequently  enormous  in  the  turbo-genera- 
tor. For  this  reason  adequate  bracing  of  the  coil  ends  is  of 
supreme  importance.  Greatest  reliance  is  placed  on  metallic 
braces  securely  bolted  to  the  generator  frame.  For  involute 
coils,  braces  may  be  used  that  consist  of  U  shaped  clevises 
which  are  thoroughly  insulated  with  treated  tape  and  bolted 
to  the  machine  frame  at  the  ends  through  spacers.  For  dia- 
mond coils,  the  braces  take  the  form  of  malleable  iron  brackets 
bolted  to  the  frame.  The  coils  should  be  rigidly  secured 
to  these  braces  by  nonmetallic  clamps,  reinforced  by  brass 
plates,  as  shown  in  Fig.  156  and  fastened  to  the  braces  by 
insulated  bolts. 

Connecting  the  Winding. — Turbine  windings  are  almost 
always  connected  with  one  group  of  coils  per  pole  per  phase. 
The  coils  of  each  group  are  connected  in  series.  In  some 
cases  these  connectors  are  riveted  as  well  as  soldered  in  place. 


REWINDING  ALTERNATING-CURRENT  MACHINES     229 

The  groups  per  phase  may  be  connected  in  parallel  or  series, 
depending  on  the  requirements  of  the  machine.  Ordinarily 
the  series  connection  is  used  on  machines  of  2200  volts  or 
higher.  The  end  connections  should  be  fully  insulated  and 
supported  on  the  rear  of  the  braces. 

Break-down  Test. — After  all  the  coils  have  been  placed 
in  the  core,  their  free  ends  should  be  connected  together  and 
the  standard  break-down  voltage  (see  page  175,  Chapter  VII) 
applied  from  the  copper  to  the  machine.  A  similar  break- 
down test  should  be  applied  between  phases  after  the  coils 
have  been  connected  into  groups.  No  further  tests  need  be 
applied  until  the  final  load  tests  are  run. 


CHAPTER  IX 

TESTING  INDUCTION  MOTOR  WINDINGS  FOR  MIS- 
TAKES AND  FAULTS 

After  the  coils  have  been  placed  in  the  stator  of  an  induction 
motor  and  connected  by  the  winder  to  give  the  required  com- 
bination'of  phases,  volts  and  poles,  the  next  step  is  to  connect 
the  motor  to  a  test  circuit  and  note  whether  or  not  the  current 
taken  is  the  same  in  all  the  leads.  If  it  is  not,  or  the  motor 
does  not  operate  properly  in  other  respects,  the  winding  must 
be  checked  and  tested  for  possible  errors  in  connection  of 
coils  and  for  other  defects.  For  an  experienced  induction 
motor  winder,  this  is  a  simple  job  and  one  that  is  done  very 
quickly.  For  the  repaiman  who  has  had  less  experience  in 
connecting  induction  motors,  the  job  is  not  so  quickly  per- 
formed. Yet  it  is  not  a  difficult  one  when  the  procedure  is 
correct.  One  of  the  most  easily  understood  methods  of  pro- 
cedure for  checking  induction-motor  windings  has  been  formu- 
lated by  A.  M.  Dudley  and  published  in  the  Electrical  Journal. 
It  is  so  simple  yet  complete  that  any  repairman  can  with  a 
little  patience  discover  and  correct  even  the  most  puzzling  of 
errors  and  defects.  It  is  given  complete  in  what  follows. 

The  errors  which  may  occur  in  induction-motor  windings 
have  been  arranged  by  Mr.  Dudley  in  the  approximate  order 
of  their  occurrence  as  follows: 

I.  Short  Circuit  and  Grounds. 

1.  Short  circuits  or  grounds  of  one  or  more  turns  in  an  individual 

coil  caused  by  a  break  in  insulation. 

2.  Short  circuit  of  a  complete  coil  caused  by  connecting  together  the 

two  ends  of  the  coil.     This  is  known  by  winders  as  "  stubbing  a 
coil  dead." 

3.  Short-circuiting  a  complete  pole-phase-group. 

4.  Short-circuiting  one  complete  phase  of  the  winding. 

II.  Reversal  of  Part  of  the  Winding. 

1.  This  may  be  confined  to  a  single  coil  or  it  may  be  a.  pole-phase- 
group  or  a  complete  phase. 

230 


TESTING  INDUCTION-MOTOR  WINDINGS 


231 


III.  Open  Circuits. 

1.  Caused  by  actually  leaving  the  winding  disconnected  or  open 

at  some  point. 
IV  Placing  One  or  More  Coils  too  Many  or  too  Few  in  a  Pole-phase-group. 

V.  Using  an  Improper  Group  Connection. 

1.  For  example,  connecting  in  series  when  the  winding  should  have 
been  in  parallel  or  vice  versa.  This  is  called  "connecting  for 
double  or  half  voltage,  respectively."  A  similar  error  would  be 
connecting  delta  instead  of  star. 

VI.  Connecting  for  the  Wrong  Number  of  Poles. 

1.  While  this  error  comes  under  No.  V,  it  is  usually  regarded  as  dif- 
ferent and  is  sometimes  difficult  to  locate  unless  it  hasbeen  noticed 
that  the  speed  of  the  motor  when  running  light  is  not  what  it 
should  be. 

Testing  for  Grounds  and  Short  Circuits. — Grounds  of  any 
description  are  located  by  "ringing  out"  between  the  copper 


Support  at  End 
of  Laminations 


FIG.  158. — Section  and  plan  views  of  a  short-circuit  detecting  device  for 
testing  stator  windings. 

and  the  frame  with  a  magneto  or  "lighting  out"  between  the 
same  points  with  an  ordinary  ItO-volt  lighting  circuit  test 
lamp.  They  are  comparatively  simple  to  locate  and  easy  tc 
repair  by  the  use  of  insulating  material  used  with  the  job. 
Short  circuits  of  a  few  turns  or  a  single  coil  become  hot  in  a 
short  time  if  the  motor  is  run  light  on  normal  voltage.  Their 
presence  can  be  detected  by  feeling  around  the  winding  with 
the  hand  immediately  after  starting  the  machine  and  noting 
if  any  coils  are  much  warmer  than  others. 


232 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


Short-circuit  Detecting  Device. — A  device  that  can  be  used 
for  detecting  short  circuits  before  the  rotor  is  placed  in  the 
stator  and  without  applying  voltage  to  the  winding  itself,  is 
shown  in  Fig.  158.  It  it  somewhat  similar  to  a  large  horse- 
shoe magnet  except  that  the  iron  part  is  built  up  of  lamina- 
tions. It  may  also  be  considered  a  core-type  transformer 
having  the  primary  coil  only  and  having  one  side  of  the  iron 
core  missing.  The  coil  is  excited  with  alternating  current  of 

a  suitable  voltage  and  then 
the  complete  device  is 
passed  slowly  around  the 
bore  of  the  stator  as  shown 
in  Figs.  159  and  160.  The 
laminations  of  the  stator 
core  complete  the  magnetic 
circuit  of  the  testing  de- 
vice and  an  alternating 
magnetic  field  flows  in  the 
stator  core  as  shown  by  the 
dotted  lines  in  Fig.  159. 
When  moving  the  device 
around  the  stator  bore,  and 
it  passes  over  a  short-cir- 
cuited turn  or  coil,  such 
short-circuited  turn  or  coil 

immediately  acts  as  a  short-circuited  secondary  coil  on  a 
transformer  of  which  the  exciting  coil  on  the  testing  device  is 
the  primary.  As  in -any  short-circuited  transformer  an  in- 
creased current  flows  both  in  the  primary  and  in  the  secondary 
coil  and  can  be  detected  by  an  ammeter  in  series  with  the 
device  or  by  the  heating  which  immediately  takes  place  in  the 
defective  coil  in  the  winding  or  by  the  attraction  which  the 
other  side  of  the  short-circuited  coil  has  for  a  strip  of  sheet 
iron.  By  passing  the  device  slowly  around  the  core  and  ob- 
serving its  behavior  from  point  to  point,  short  circuits  can 
readily  be  detected.  This  refers  particularly  to  short  circuits 
in  individual  turns  or  in  one  complete  coil.  A  short  circuit  of 
a  complete  pole-phase-group  is  more  readily  located  by  a  com- 
pass test,  and  a  short  circuit  of  an  entire  phase  can  be  found 


FIG.  159. — Method  of  using  the  short- 
circuit  detecting  device  shown  in  Fig. 
158. 


TESTING  INDUCTION-MOTOR  WINDINGS  233 

by  a  balance  test.  The  balance  test  is  made  after  the  winding 
has  been  checked  for  grounds  and  short-circuited  turns  and 
has  had  the  resistance  of  all  the  phases  measured.  If  these 
checks  indicate  the  proper  number  of  turns  in  series,  a  com- 
paratively low  alternating  voltage  is  applied  to  the  winding 
of  the  stator  without  the  rotor  being  in  place.  The  current  is 
then  read  in  all  the  phases  and  if  it  checks  up  the  same  or 
balances  as  it  is  called,  the  machine  is  considered  free  of 
grounds  and  short  circuits. 


FIG.  160. — Testing  stator  winding  for  short-circuited  coils  by  use  of  a  small 
alternating-current  magnet. 

Reversal  of  One  or  More  Coils  or  Groups. — It  happens  that 
individual  coils  or  sometimes  entire  groups  are  connected  in 
backward.  If  the  error  is  confined  to  one  coil  it  does  not  usu- 
ally show  up  on  the  "  balance "  test  and,  of  course,  would  not 
be  found  on  the  resistance  test  since  the  resistance  is  the  same 
no  matter  whether  the  ends  of  the  coils  are  interchanged  or 
not.  Such  reversed  coils  or  groups  can  be  located  by  means  of 
a  polarity  test  with  a  compass.  In  this  test  the  motor  wind- 


234         ARMATURE  WINDING  AND  MOTOR  REPAIR 

ings  are  excited  by  a  comparatively  low  direct-current  voltage 
so  chosen  that  the  current  is  limited  to  a  reasonable  value. 
If  the  windings  are  so  excited  and  a  compass  is  placed  inside 
the  bore  and  passed  around  slowly  following  the  inner  periph- 
ery, the  needle  of  the  compass  will  reverse  in  passing  from 
a  north  pole  to  a  south  pole  group  and  vice  versa.  If  an  indi- 
vidual coil  is  reversed  it  will  show  a  tendency  to  reverse  the 
compass  needle  when  the  needle  is  directly  over  that  coil.  If 
an  entire  pole-phase-group  is  reversed  the  compass  needle  will 
indicate  the  same  direction  of  field  on  two  successive  groups. 
Also  if  a  coil  is  left  out  of  circuit  or  is  "dead,"  as  already  men- 
tioned, it  will  indicate  an  irregularity  at  the  instant  the  com- 
pass passes  directly  above  it.  By  checking  the  three  phases 
of  a  three-phase  winding  separately,  in  this  manner  and 
marking  the  result  inside  the  bore  with  chalk  of  different 
colors,  it  is  possible  to  check  for  the  reversal  of  an  entire  phase. 

Open  circuits  are  best  checked  by  "  lighting  out."  This 
test  it  made  by  connecting  a  110-volt  lighting  circuit  and  an 
incandescent  lamp  in  series  with  the  winding.  If  there  is  an 
open  circuit  the  lamp  will  not  light. 

Placing  the  wrong  number  of  coils  in  two  or  more  phase 
groups  can  hardly  be  deteced  otherwise  than  by  actually 
making  a  physical  count.  Since  this  is  a  simple  matter,  the 
check  is  best  made  in  that  way. 

Using  an  improper  group  connection  has  in  general  the 
same  effect  as  raising  or  lowering  the  voltage  on  a  machine 
and  can  best  be  checked  by  operating  the  motor  on  what 
should  be  the  correct  voltage.  If  this  voltage  is  considerably 
too  high  as  would  be  the  case  if  the  winding  was  connected 
parallel  when  it  should  be  series,  or  delta  when  it  should  be 
star,  the  motor  will  emit  more  than  the  ordinary  amount  of 
magnetic  hum  and  will  probably  overheat  in  a  short  time  even 
when  not  mechanically  loaded.  If  the  voltage  is  much  too 
low,  as  would  be  the  case  if  the  winding  was  connected  series, 
when  it  should  be  parallel,  or  star  when  it  should  be  delta, 
the  fact  can  be  determined  by  trying  a  load  on  the  motor. 
When  the  load  comes  on  the  drop  in  speed  will  be  too  great 
and  the  motor  will  pull  out  and  come  to  a  standstill  on  a  load 
much  less  than  normal. 


TESTING  INDUCTION-MOTOR  WINDINGS  235 

Connecting  for  the  wrong  number  of  poles  can  most  readily 
be  detected  by  checking  the  no-load  speed.  (If  it  has  a  wound 
rotor,  this  must  be  short-circuited.)  The  no-load  speed,  being 
approximately  synchronous,  is  very  nearly  equal  to  cycles 
X  120  -r-  number  of  poles.  If  this  gives  a  result  differing 
from  that  expected,  the  winding  is  connected  for  the  wrong 
number  of  poles. 

Order  in  Which  Tests  Should  be  Made. — The  order  in  which 
these  various  checks  should  be  performed  is  usually  as  follows : 
After  the  winder  has  completed  the  connection,  the  windings 
are  checked  against  the  winding  diagram.  The  coils  per 
group  are  counted  and  a  visual  inspection  made  for  short 
circuits,  open  circuits  and  reversed  coils.  A  balance  test  is 
made  with  low  voltage  to  see  if  the  separate  phases  show  the 
same  result.  A  high  voltage  test  is  then  made  on  the  insula- 
tion to  ensure  that  the  coils  are  not  grounded  between  phases 
nor  on  the  iron  core.  The  machine  is  then  assembled  and  the 
resistance  of  the  completed  winding  is  measured  on  all  phases. 
If  these  checks  are  satisfactory  the  machine  can  be  passed  for  a 
running  light  test  without  load.  Sufficient  voltage  is  put  on 
the  windings  to  start  the  motor  up.  If  it  comes  up  to  speed 
without  apparent  distress  or  irregularity  of  any  kind,  the  speed 
is  checked  and  the  temperature  of  the  winding  is  tested  with 
the  hand  all  the  way  around  the  machine.  If  this  is  normal 
the  voltage  is  raised  to  its  normal  value  and  the  no-load  cur- 
rent in  all  phases  and  the  watts  are  read.  If  these  values 
check  with  previous  calculations  or  tests  on  duplicate  machines 
the  windings  are  considered  to  be  correctly  connected.  If  no 
data  is  available  on  the  no-load  current  it  may  be  considered 
reasonable  if  it  does  not  exceed  40  per  cent,  and  is  at  least  20 
per  cent,  of  full  load  current.  The  no-load  watts  running 
light  may  be  considered  reasonable  if  they  are  roughly  in 
the  neighborhood  of  seven  or  eight  per  cent,  of  the  normal 
rating  of  the  motor.  If  the  motor  does  not  readily  come  up 
to  speed  or  the  phases  do  not  balance  or  there  are  signs  of 
unequal  heating  in  the  winding  or  other  distress,  the  rotor 
should  be  removed  and  the  connections  checked.  If  the  error 
is  not  apparent  and  a  source  of  direct  current  is  available  the 
compass  test  may  be  applied. 


236         ARMATURE  WINDING  AND  MOTOR  REPAIR 

Connections  for  Applying  Direct  Current  when  Exploring 
A.-C.  Windings  with  a  Compass. — The  accompanying  dia- 
grams show  the  methods  for  making  connections  for  the  use 
of  direct  current  to  excite  the  windings  of  an  alternating-current 
motor  so  as  to  explore  the  windings  by  the  use  of  a  compass. 
When  the  winding  is  star  connected,  one  lead  of  the  direct- 
current  circuit  is  joined  to  A-B-C  connected  together  and 


D.C.Supgly> 


FIG.  161. — Method  for  checking  3-phase,  star  and  delta  connected  windings 
with  direct  current. 

the  other  lead  to  the  neutral  point.  When  the  compass  is 
held  over  the  coils  in  the  slots  and  moved  around  the  stator, 
there  should  be  three  times  as  many  poles  as  there  are  poles 
in  the  machines  reversing  alternately  and  spaced  equally  when 
the  winding  is  balanced.  In  case  of  a  delta-connected  winding, 
it  is  only  necessary  to  open  the  delta  connection  as  shown  in 
Fig.  161  at  the  right  and  connect  the  two  ends  to  the  direct- 
current  leads. 


CHAPTER  X 

ADAPTING  DIRECT-CURRENT  MOTORS  TO  CHANGED 
OPERATING  CONDITIONS 

The  changes  in  direct-current  motors  which  the  repairman 
is  most  frequently  called  upon  to  make  are:  1.  Changes  in 
speed.  2.  Changes  in  operating  voltage.  3.  Changes  to 
operate  a  motor  as  a  generator  and  vice  versa. 

Changes  in  Speed. — A  change  in  speed  of  a  motor  from  10 
to  15  per  cent,  can  usually  be  made  by  increasing  or  decreas- 
ing the  air  gap  between  the  armature  and  fields.  In  general 
the  per  cent,  changes  in  air  gap  required  will  be  three  or  four 
times  the  per  cent,  change  in  speed.  This  will  serve  only  as 
a  rough  check  for  use  when  measuring  the  air  gap  and  consider- 
ing  the  change  of  speed  in  this  manner.  To  make  this  change 
with  accuracy,  it  is  necessary  to  have  a  magnetization  curve 
for  the  design  of  the  particular  machine  dealt  with.  How- 
ever if  the  change  in  speed  desired  is  not  too  great,  the  change 
in  air  gap  will  in  most  cases  give  desired  results.  The  excep- 
tions are  those  cases  where  the  motor  has  been  built  to  work 
high  up  on  the  saturation  curve  where  the  iron  is  practically 
saturated  or  very  low  down  on  the  saturation  curve  where  the 
excitation  is  practically  all  used  in  the  air  gap.  The  speed  is 
increased  by  increasing  the  air  gap  and  reduced  by  reducing 
the  air  gap. 

It  is  usually  easier  to  reduce  the  speed  by  change  in  air  gap 
than  to  increase  the  speed.  In  the  former  case  "sheet  steel 
liners  or  shims  can  be  inserted  next  to  the  frame  and  different 
lengths  of  air  gaps  secured  over  a  considerable  range  depending 
upon  the  length  of  the  original  air  gap  of  the  machine.  If 
the  air  gap  cannot  be  increased  by  removing  such  shims,  it 
may  be  necessary  to  grind  off  the  required  amount  from  the 
pole  faces.  Such  a  grinding  should  not  be  done  more  than 
once  if  the  operation  will  narrow  the  width  of  pole  face  to  an 
appreciable  extent.  When  the  increase  in  speed  is  more  than 

237 


238         ARMATURE  WINDING  AND  MOTOR  REPAIR 

can  be  secured  by  increasing  the  air  gap,  it  may  be  possible  to 
add  a  resistance  to  the  shunt-field  circuit.  In  such  a  case  it 
will  be  necessary  to  check  carefully  the  no-load  and  full-load 
speeds. 

In  the  case  of  a  shunt  motor,  a  compound  characteristic 
can  be  secured  by  increasing  the  air  gap  and  then  adding  series 
turns  to  the  field  poles  until  the  full-load  speed  is  the  same  as 
it  was  before  the  change.  For  this  purpose  flexible  copper 
cable  can  be  wound  around  the  shunt  coil  and  bound  in  place. 
In  reversing  this  change,  that  is,  in  changing  a  compound 
motor  to  a  shunt  characteristic,  the  air  gap  should  be  reduced 
and  current  shunted  out  of  the  series  coils  of  the  field  until  a 
constant  speed  range  is  secured. 

An  inefficient  and  emergency  method  of  reducing  the  speed 
of  a  motor  for  a  short  time  only  is  by  the  use  of  a  resistance 
in  the  armature  circuit.  When  a  variable  resistance  is 
connected  in  series  with  the  armature  leads,  it  simply  reduces 
the  impressed  voltage  on  the  armature  winding  by  using  up  a 
part  of  the  voltage  in  the  resistance.  Since  the  speed  of  the 
motor  is  proportional  to  the  operating  voltage,  the  desired 
speed  reduction  can  be  thus  secured. 

In  all  cases  where  the  speed  of  a  motor  is  increased,  atten- 
tion must  be  paid  to  the  armature  construction  so  that  the 
increased  centrifugal  strains  on  the  coils  and  the  banding  due 
to  the  increase  in  speed  will  develop  no  serious  defects,  such 
as  rubbing  of  coils  on  the  fields,  high  commutator  bars  and  poor 
commutation.  When  the  increase  in  speed  is  50  per  cent,  or 
more,  the  manufacturer  should  be  consulted  for  advice  as 
to  the  safety  of  the  high  speed  for  continuous  operation. 

Changes  in  Operating  Voltage. — The  speed  of  a  direct- 
current  motor  varies  directly  with  the  operating  voltage  and 
(theoretically)  inversely  with  the  flux  of  the  fields.  When 
the  operating  voltage  is  increased  on  a  motor,  the  excitation 
of  the  fields  is  also  affected  but  on  account  of  the  saturation 
of  the  iron,  the  field  flux  is  not  affected  in  direct  proportion 
so  that  the  speed  of  the  motor  on  an  increase  of  voltage  follows 
about  in  proportion  to  the  increase.  When  the  voltage  is 
not  increased  or  decreased  more  than  25  per  cent,  over  the 
rated  value  for  the  motor,  the  correct  speed  can  be  obtained  by 


CHANGES  IN  DIRECT-CURRENT  MOTORS  239 

a  change  of  the  air  gap.  In  such  a  change  the  operating 
temperature  of  the  field  coils  must  be  watched,  since  the 
temperature  will  vary  about  as  the  square  of  the  voltage. 
A  temperature  higher  than  160°F.  will  injure  the  insula- 
tion. To  prevent  this  when  the  temperature  of  the  fields 
is  found  to  be  around  this  figure,  it  will  be  advisable 
to  rewind  the  field  coils.  In  case  the  machine  is  operated 
on  under  voltage  with  the  air  gap  as  large  as  possible,  a  re- 
sistance can  be  used  in  the  shunt  field  until  the  required  speed 
is  secured.  On  a  compound  motor,  no-load  and  full-load 
speeds  can  be  adjusted  by  changing  the  series  field  coils  in 
either  of  the  cases  mentioned. 

Operating  a  Motor  on  One-half  or  Double  Voltage. — 
It  frequently  happens  that  a  220-volt  motor  must  be  changed 
to  operate  on  110  volts  or  a  110-volt  motor  on  220  volts. 
In  the  case  of  a  220-volt  motor,  it  is  usually  possible  to  connect 
the  shunt-field  coils  in  two  groups  and  then  connect  these 
groups  in  parallel.  In  such  a  case  when  the  motor  is  connected 
to  a  110-volt  circuit,  the  voltage  per  coil  will  be  the  same  so 
that  the  field  flux  will  be  the  same,  but  the  speed  will  be  only 
one-half  that  on  220  volts.  The  speed  can  be  brought  up 
to  normal  by  increasing  the  air  gap  as  much  as  possible  and 
using  resistance  in  the  shunt  field  as  explained  in  a  preceding 
paragraph.  The  rating  in  horsepower  of  a  motor  so  changed 
will  only  be  one-half  of  what  it  was  before. 

When  a  110-volt  motor  must  be  changed  to  operate  on  220 
volts,  the  conditions  are  not  so  easy  as  in  the  previous  case 
for  if  the  smallest  air  gap  with  the  shunt  fields  in  series  cannot 
be  used,  the  field  coils  must  be  rewound.  When  operated 
in  this  way  the  motor  will  have  double  the  horsepower  rating 
on  220  volts  that  it  did  on  110  volts.  The  changes  in  the 
motor  which  have  been  mentioned  do  not  require  a  change 
in  the  armature  winding. 

Changes  in  Armature  Winding  for  Operating  Motors  on 
One-half  or  Double  Voltage. — The  most  satisfactory  way 
of  changing  a  motor  to  operate  on  a  voltage  which  is  one-half 
or  double  the  rated  voltage  is  to  rewind  the  armature  to  suit 
the  new  conditions.  This  is  also  the  most  expensive  way 
and  need  not  be  resorted  to  when  the  change  in  voltage  is 


240         ARMATURE  WINDING  AND  MOTOR  REPAIR 

not  over  25  per  cent.  A  direct-current  armature  winding  can 
usually  be  changed  for  operation  on  a  lower  voltage  in  two 
ways.  1.  By  using  new  coils.  2.  By  reconnecting  the  old 
armature  winding.  When  the  armature  is  to  be  rewound 
for  a  different  voltage,  the  number  of  turns  in  series  between 
brushes  will  vary  directly  as  the  old  voltage  to  the  new.  The 
cross-section  of  the  wire  for  the  coils  will  also  vary  inversely 
as  the  old  voltage  to  the  new.  For  instance  if  a  110-volt 
motor  is  to  be  operated  on  220  volts,  the  armature  conductors 
will  be  doubled  in  number  and  their  cross-section  made  one- 
half,  when  the  speed  and  horsepower  rating  will  remain  the 
same  as  before.  When  the  size  of  wire  previously  used  is 
known,  one  of  one-half  the  cross-section  will  be  three  B.  &  S, 
gauge  numbers  higher. 

A  practical  way  of  making  these  changes  in  armature 
windings  in  a  repair  shop  is  given  on  pages  243  to  260  as  out- 
lined by  T.  Schutter  (Electrical  Engineering,  July  and  August, 
1918)  for  the  changes  of  one -half  and  double- volt  age  operation. 

Size  of  Wire  for  D.-C.  Armature  Coils. — Before  winding 
the  coils  needed  in  the  repair  of  an  armature  it  must  be  decided 
whether  a  lap  or  a  wave  winding  will  be  used.  In  the  lap 
winding  there  are  as  many  current  paths  or  circuits  through 
the  armature  winding  as  there  are  poles  with  the  coils  con- 
nected in  series  in  each  of  these  paths  or  circuits.  The  wave 
winding  has  only  two  current  paths  through  the  armature 
winding  regardless  of  the  number  of  poles  the  machine  has. 
In  each  of  these  two  paths  the  coils  are  connected  in  series. 
Considering  /  the  total  armature  current  as  given  on  the  name 
plate  of  the  machine  and  i  the  current  in  each  circuit  of  the 
armature  winding,  the  following  formulas  can  be  used  to  find 
the  maximum  current  that  can  be  carried  by  the  coils  of 
each  circuit  connected  in  series. 

For  a  lap  winding  in  which  the  number  of  circuits  equals 
the  number  of  poles  or  p, 


For  a  multiple  lap  winding,  having  two  or  more  single  lap 
windings  in  parallel, 

i  =  I  -T-  mp 


CHANGES  IN  DIRECT-CURRENT  MOTORS  241 

In  this  case  m  is  the  number  of  single  lap  windings  in  parallel. 

For  a  wave  winding  which  has  two  circuits 
i  =  I  +  2 

For  wave  windings  having  more  than  two  circuits,  usually 
called  multiple-wave  or  series-parallel  windings 

i  =  I  -f-  2m 

In  this  case  m  is  the  number  of  wave  windings  used  in  the 
armature. 

The  permissible  current  density  in  armature  conductors 
varies  from  1500  to  3000  amperes  per  square  inch.  The 
cross-section  or  diameter  of  the  wire  for  the  armature  coils 
can  then  be  found  by  dividing  the  current  in  each  section 
of  the  winding  (i)  by  the  allowable  amperes  per  square  inch. 
The  value  to  use  in  the  case  of  small  machines  should  not 
exceed  3000;  in  intermediate  sizes  2000;  and  in  large  sizes 
1500. 

When  winding  new  coils  in  the  repair  of  an  armature  it  is 
necessary  to  determine  the  available  winding  space  in  the 
slot  before  deciding  upon  the  number  of  turns  and  form  of 
coil  to  use.  In  case  the  armature  is  to  be  rewound  for  changed 
conditions  of  speed  or  voltage  reference  should  be  made  to 
Chapters  X  and  XI  where  details  to  be  observed  are  given 
for  different  changes  in  operating  conditions. 

Operating  a  Generator  as  a  Motor  and  Vice  Versa. — It  is 
usually  possible  to  operate  a  generator  as  a  motor  by  setting 


FIG.  162. — Direct-current  generator  with  armature  rotating  clockwise. 

the  brushes  for  the  correct  rotation  of  the  armature  and  with 
a  backward  lead  instead  of  a  forward  lead  when  operating  as 
a  generator.  To  successfully  operate  a  motor  as  a  generator 
the  air  gap  should  be  reduced  to  a  minimum  and  the  speed 

16 


242         ARMATURE  WINDING  AND  MOTOR  REPAIR 

increased  where  the  voltage  as  a  generator  is  to  be  equal  or 
higher  than  the  voltage  as  a  motor.  When  the  voltage  is  to 
be  lower,  the  same  speed  and  air  gap  can  usually  be  used. 
When  a  direct-current  machine  is  changed  from  a  generator 
to  a  motor,  the  current  in  both  the  armature  and  series  field 
reverses.  If,  therefore,  the  machine  is  operated  as  a  cumu- 
latively compounded  generator,  it  will  also  operate  as  a  differ- 
entially compounded  motor.  In  order  to  use  a  compound 
generator  as  a  motor,  it  is  usually  necessary  to  reverse  the 
connections  of  its  series  fields.  The  diagrams  of  Figs.  162 
and  163  are  self-explanatory. 


Fio.  163. — Differential   motor  with  armature  rotating  clockwise  and  series 
field  opposing  shunt  field. 

Adjusting  the  Air  Gap  on  D.-C.  Machines. — When  insert- 
ing the  armature  in  any  machine  in  which  the  bearings  are 
independent  of  the  frame,  the  air  gap  between  the  armature 
core  and  pole  faces  should  be  checked  up.  Any  inequality 
of  air  gap  will  cause  unnecessary  friction  and  heating  of  the 
bearings  and  an  unequal  heating  of  the  armature  iron.  The 
air  gap  may  be  adjusted  horizontally  in  many  cases  by  cross 
beams  and  jack  screws  on  the  bedplate  and  vertically  by 
thin  sheet  liners  inserted  between  the  bedplate  and  the 
yoke.  The  air  gap  can  be  gauged  during  the  operation  by 
inserting  a  hardwood  wedge  on  the  front  and  back  ends  of 
the  machine  at  different  points,  noting  the  distance  to  which 
the  wedge  enters  each  time.  The  adjustment  should  be  con- 
tinued until  the  air  gap  is  the  same  around  the  entire  circum- 
ference of  the  machine. 

Motor  Speed  when  Reconnecting  a  D.-C.  Motor  Winding 
Wave  to  Lap. — When  reconnecting  a  wave  winding  to  form 
a  lap  winding  without  changing  the  coils,  or  voltage,  the  speed 


CHANGES  IN  DIRECT-CURRENT  MOTORS  243 

of  a  four-pole  motor  will  be  increased  twice,  and  the  speed 
of  an  eight-pole  motor  increased  four  times.  This  is  because 
the  wave  winding  is  a  two-circuit  winding  while  the  lap 
winding  has  as  many  circuits  as  there  are  poles.  The 
speed,  therefore,  varies  directly  as  the  number  of  circuits  in 
the  winding. 

Change  in  Brushes  when  Reconnecting  a  D.-C.  Motor 
from  a  Higher  to  a  Lower  Voltage. — When  reconnecting  a 
direct-current  armature  from  a  higher  to  a  lower  voltage, 
say  220  volts  to  110  volts,  the  size  of  conductors  must  be 
doubled  as  the  current  will  be  about  double  its  original  value. 
The  original  brushes  on  the  220-volt  machine  must  therefore 
be  changed  to  handle  this  increase  in  current  without  heating 
and  sparking.  If  twice  as  many  brushes  of  the  same  size  as 
before  cannot  be  used,  the  size  of  brush  must  be  changed  to 
give  twice  the  bearing  surface.  An  increase  in  width  of  the 
brush  should  be  avoided,  the  length  being  increased  to  get  the 
proper  bearing  surface.  A  current  density  of  more  than  50 
amperes  per  square  inch  will  give  trouble  sooner  or  later  in 
a  reconnected  armature. 

For  a  four-pole  motor  originally  220  volts,  changed  to 
operate  on  110  volts,  the  field  coils  should  be  connected  two 
in  series  and  these  groups  connected  in  parallel  in  order  to 
get  the  same  field  current. 

REWINDING  AND  RECONNECTING  DIRECT- CURRENT  ARMA- 
TURE WINDINGS  FOR  A  CHANGE  OF  VOLTAGE* 

When  a  direct-current  armature  is  to  be  rewound  or  re- 
connected for  a  change  from  one  voltage  to  another,  the 
number  of  turns  in  series  between  brushes  as  explained  on  page 
239  will  vary  directly  as  one  voltage  to  the  other  and  the 
cross-sectional  area  of  the  wire  will  vary  inversely  as  the  vol- 
tages. To  illustrate,  take  the  case  of  a  2-pole  armature  con- 
sisting of  24  coils,  20  turns  per  coil  wound  one  wire-in-hand 
using  number  19  B.  &  S.  gauge  wire,  which  has  a  cross-sec- 
tional area  of  1290  circular  mils.  The  armature  has  a  240-volt 
winding  and  is  to  be  changed  so  as  to  operate  on  a  120- volt 

*  T,  Schutter,  Electrical  Engineering,  July,  1918, 


244         ARMATURE  WINDING  AND  MOTOR  REPAIR 

circuit.  This  can  be  accomplished  in  two  ways.  First,  by 
rewinding  the  armature;  second,  by  reconnecting  the  present 
winding. 

By  comparing  the  original  voltage  with  the  new  operating 
voltage,  it  will  be  seen  that  the  new  voltage  is  just  half  of  the 
original  voltage.  If  it  took  20  turns  per  coil  for  240  volts 
then  it  will  take  half  of  20  or  10  turns  per  coil  for  120  volts. 
As  the  machine  is  to  do  the  same  amount  of  work,  it  will 
carry  twice  the  current  at  120  volts  that  it  did  at  240  volts. 
For  this  reason  the  wire  must  have  twice  the  cross-sectional  area, 
or  1290  X  2  =  2580  circular  mil  area.  This  is  equal  to  a  num- 
ber 16  B.  &  S.  gauge  wire,  still  using  the  same  number  of  coils 
as  before. 


Original  240  volt  connection 

FIG.  164. — A  240- volt  lap  winding  which  is  reconnected  for  120  volts  as 
shown  in  Fig.  165. 

The  other  method  is  to  reconnect  the  winding  so  that  two 
coils  are  in  parallel,  and  bridge  two  commutator  bars.  This 
will  result  in  a  winding  of  12  coils,  two  in  parallel,  with  20 
turns  per  coil.  In  Fig.  164  the  original  winding  is  shown,  as 
it  was  wound  and  connected  to  the  commutator.  The  arma- 
ture core  contained  24  slots  and  there  were  two  coil  sides  per 
slot.  The  part  of  the  slot  which  is  occupied  by  a  coil  side  is 
called  a  winding  space.  The  odd  numbered  winding  spaces 
are  considered  as  being  in  the  bottom  of  the  slot  and  the  even 
numbered  winding  spaces  being  in  the  top  of  the  slot.  The 
table  for  winding  coils  in  Fig.  164  is  given  on  page  245  and 
the  connecting  table  for  the  original  winding  as  illustrated  in 
Fig.  164  on  page  247. 


CHANGES  IN  DIRECT-CURRENT  MOTORS 


245 


By  tracing  the  direction  of  flow  of  current  through  the  wind- 
ings from  the  positive  brush  to  the  negative  brush,  it  will  be 
seen  there  are  two  paths  or  circuits,  each  consisting  of  12  coils 
in  series,  the  two  paths  or  circuits  being  in  parallel  with  each 
other.  This  winding  operates  on  240  volts. 

TABLE  FOR  WINDING  COILS  ON  ARMATURE  OF  FIG.  164 


Coil  number 

Coils  are  wound 

In  spaces  number 

In  slots  number 

1 

1  and  24 

1  and  12 

2 

3  and  26 

2  and  13 

3 

5  and  28 

3  and  14 

4 

7  and  30  . 

4  and  15 

5 

9  and  32 

5  and  16 

6 

11  and  34 

6  and  17 

7 

13  and  36 

7  and  18 

8 

15  and  38 

8  and  19 

9 

17  and  40 

9  and  20 

10 

19  and  42 

10  and  21 

11 

21  and  44 

11  and  22 

12 

23  and  46 

12  and  23 

13 

25  and  48 

13  and  24 

14 

27  and    2 

Hand    1 

15 

29  and    4 

15  and    2 

16 

31  and    6 

16  and    3 

17 

33  and    8 

17  and    4 

18 

35  and  10 

18  and    5 

19 

37  and  12 

19  and    6 

20 

39  and  14 

20  and    7 

21 

41  and  16 

21  and    8 

22 

43  and  18 

22  and    9 

23 

45  and  20 

23  and  10 

24 

47  and  22 

24  and  11 

246         ARMATURE  WINDING  AND  MOTOR  REPAIR 

TABLE  FOR  CONNECTING  COILS  OP  FIG.  164  TO  COMMUTATOR 


Beginning  of  coil 
number 

To  bar  number 

End  of  coil  number 

To  bar  number 

1 

6 

1 

7 

2 

7 

2 

8 

3 

8 

3 

9 

4 

9 

4 

10 

5 

10 

5 

11 

6 

11 

6 

12 

7 

12 

7 

13 

8 

13 

8 

14 

9 

14 

9 

15 

10 

15 

10 

16 

11 

16 

11 

17 

12 

17 

12 

18 

13 

18 

13 

19 

14 

19 

14 

20 

15 

20 

15 

21 

16 

21 

16 

22 

17 

22       >' 

17 

23 

18 

23 

18 

24 

19 

24 

19 

1 

20 

1 

20 

2 

21 

2 

21 

3 

22 

3 

22 

4 

23 

4 

23 

5 

24 

5 

24 

6 

PIG.  165. — The  240-volt  winding  of  Fig.   164  reconnected  for  a  120-volt 
circuit  by  connecting  two  coils  in  parallel  and  using  wider  brushes. 

Reconnecting  a  Lap  Winding. — In  Fig.  165  the  winding 
shown  in  Fig.  164  has  been  reconnected  so  as  to  operate  on 
a  120-volt  circuit. 


CHANGES  IN  DIRECT-CURRENT  MOTORS 


247 


As  explained  it  will  require  one-half  as  much  winding  on  120 
volts  as  it  did  on  240  volts.  This  can  be  accomplished  by 
connecting  two  coils  in  parallel,  and  using  wider  brushes; 
that  is,  the  brushes  should  be  at  least  as  wide  as  1J/2  commu- 
tator bars  and  not  more  than  two  commutator  bars.  If  it 
is  possible  to  arrange  for  the  use  of  the  wider  brushes,  the 
commutator  can  be  bridged,  as  shown  by  the  jumpers  A,  B,  C, 
etc.,  Fig.  165. 

v         'I'' 

TABLE  FOB  CONNECTING  COILS  OF  FIG.  165  TO  COMMUTATOR 


Beginning  of  coil 
number 

To  bar  number 

End  of  coil  number 

To  bar  number 

1 

5 

1 

7 

2 

6 

2 

8 

3 

7 

3 

9 

4 

8 

4 

10 

5 

9 

5 

11 

6 

10 

6 

12 

7 

11 

7 

13 

8 

12 

8 

14 

9 

13 

9 

15 

10 

14 

10 

16 

11 

15 

11 

17 

12 

16 

12 

18 

13 

17 

13 

19 

14 

18 

14 

20 

15 

19 

15 

21 

16 

20 

16 

22 

17 

21 

17 

23 

18 

22 

18 

24 

19 

23 

19 

1 

20 

24 

20 

2 

21 

1 

21 

3 

22 

2 

22 

4 

23 

3 

23 

5 

24 

4 

24 

6 

By  tracing  the  direction  of  flow  of  current  through  the  wind- 
ing it  will  be  seen  that  the  current  flows  in  four  circuits  from 
the  positive  brush  to  the  negative  brush.  In  each  path  or  cir- 
cuit there  are  six  coils  in  series,  and  the  four  paths  or  circuits 
are  in  parallel.  This  reconnection  is  equivalent  to  rewinding 


248         ARMATURE  WINDING  AND  MOTOR  REPAIR 


the  armature  with  twice  the  size  of  wire  and  one-half  the 
number  of  turns  with  which  it  was  wound  for  240  volts.  Both 
Figs.  164  and  165  show  lap  or  parallel  windings. 

Reconnecting  a   Wave   Winding. — Fig.    166   represents   a 
four-pole  wave  or  series  winding,  consisting  of  31  .coils.     The 


Slot  No. 


Original240volt  Connection 

FIG.   166. — Wave  winding  for  a  240-volt,  4-pole  armature. 

armature  core  has  31  slots,  and  there  are  two  coil  sides  per 
slot.  It  is  wound  so  as  to  be  operated  on  a  240-volt  circuit. 
The  coils  in  this  Fig.  166  are  placed  so  that  the  beginning  of 
each  coil  is  placed  in  the  bottom  of  the  slot,  and  is  represented 
by  the  odd  numbered  coil  sides. 


Winding 
Space  No. 


Slot  No. 


Reconnected  for  120  Volts 

-  FIG.  167.— The  wave  winding  of  Fig.  166  reconnected  so  that  there  are 
four  paths  for  current  instead  of  two  with  four  brushes  instead  of  two.  This 
winding  can  now  be  used  on  a  120-volt  circuit. 

By  tracing  the  current  from  the  positive  to  the  negative 
brush,  it  will  be  seen  that  the  winding  is  divided  into  two  paths 
or  circuits.  To  change  this  winding  so  that  it  can  be  operated 
on  a  120-volt  circuit,  two  methods  can  be  used :  First,  reconnect 
the  winding  so  that  it  will  consist  of  four  paths  or  circuits, 
and  add  another  set  of  brushes  as  shown  in  Fig.  167.  The 


CHANGES  IN  DIRECT-CURRENT  MOTORS 
TABLE  FOR  WINDING  COILS  IN  SLOTS  OF  FIG.  166 


249 


Coil  number 

Coils  are  wound 

In  spaces  number 

In  slots  number 

1 

1  and  16 

1  and  8 

2 

3  and  18 

2  and  9 

3 

5  and  20 

3  and  10 

4 

7  and  22 

4  and  11 

5 

9  and  24 

5  and  12 

6 

11  and  26 

6  and  13 

7 

13  and  28 

7  and  14 

8 

15  and  30 

8  and  15 

9 

17  and  32 

9  and  16 

10 

19  and  34 

'  10  and  17 

11 

21  and  36 

11  and  18 

12 

23  and  38 

12  and  19 

13 

25  and  40 

13  and  20 

14 

27  and  42 

14  and  21 

15 

29  and  44 

15  and  22 

16 

31  and  46 

16  and  23 

17 

33  and  48 

17  and  24 

18 

35  and  50 

18  and  25 

19 

37  and  52 

19  and  26 

20 

39  and  54 

20  and  27 

21 

41  and  56 

21  and  28 

22 

43  and  58 

22  and  29 

23 

45  and  60 

23  and  30 

24 

47  and  62 

24  and  31 

25 

49  and  2 

25  and  1 

26 

51  and  4 

26  and  2 

27 

53  and  6 

27  and  3 

28 

55  and  8 

28  and  4 

29 

57  and  10 

29  and  5 

30 

59  and  12 

30  and  6 

31 

61  and  14 

31  and  7 

additional  brushes  are  marked  AI  and  B\.  Second,  connect 
two  coils  in  parallel  and  still  have  two  circuits  in  the  winding. 
This,  however,  will  necessitate  the  dropping  of  one  coil,  as 
shown  by  the  heavy  lines  in  Fig.  168.  Only  two  brushes 
will  be  required  as  shown,  if  they  can  be  set  so  that  they 
cover  at  least  1J^  bars,  the  jumpers  A,  B,  C,  etc.,  can  be 
omitted. 


250         ARMATURE  WINDING  AND  MOTOR  REPAIR 

TABLE  FOR  CONNECTING  COILS  OF  FIG.  166  TO  THE  COMMUTATOR 


Beginning  of  coil 
number 

To  bar  number 

End  of  coil  number 

To  bar  number 

1 

28 

1 

12 

2 

29 

2 

13 

3 

30 

3 

14 

4 

31 

4 

15 

5 

1 

5 

16 

6 

2 

6 

17 

7 

3 

7 

18 

8 

4 

8 

19 

9 

5 

9 

20 

10 

6 

10 

21 

11 

7 

11 

22 

12 

8 

12 

23 

13 

9 

13 

24 

14 

10 

14 

25 

15 

11 

15 

26 

16 

12 

16 

27 

17 

13 

17 

28 

18 

14 

18 

29 

19 

15 

19 

30 

20 

16 

20 

31 

21 

17 

21 

1 

22 

18 

22 

2 

23 

19 

23 

3 

24 

20 

24 

4 

25 

21 

25 

5 

26 

22 

26 

6 

27 

23 

27 

7 

28 

24 

28 

8 

29 

25 

29 

9 

30 

26 

30 

10 

31 

27 

31 

11 

The  winding  as  shown  in  Fig.  167,  is  a  four-pole  lap  or 
parallel  winding,  with  four  circuits  or  paths  through  the 
winding.  The  second  method  of  reconnecting  Fig.  166  so  as 
to  operate  it  on  120  volts  is  shown  in  Fig.  168.  As  previously 
explained,  each  two  adjacent  coils  will  be  connected  in  parallel, 
and  since  there  are  31  coils  on  the  entire  winding,  one  coil 
must  be  dropped.  In  this  winding  (Fig.  168)  coil  No.  2,  shown 
with  the  heavy  lines  has  no  connection  with  the  commutator 


CHANGES  IN  DIRECT-CURRENT  MOTORS  251 

TABLE  FOR  CONNECTING  COILS  OF  FIG.  167  TO  THE  COMMUTATOR 


Beginning  of  coil 
number 

To  bar  number 

End  of  coil  number 

To  bar  number 

1 

4 

1 

5 

2 

5 

2 

6 

3 

6 

3 

7 

4 

7 

4 

8 

5 

8 

5 

9 

6 

9 

6 

10 

7 

10 

7 

11 

8 

11 

8 

12 

9 

12 

9 

13 

10 

13 

10 

14 

11 

14 

11 

15 

12 

15 

12 

16 

13 

16 

13 

17 

14 

17 

14 

18 

15 

18 

15 

19 

16 

19 

16 

20 

17 

20 

17 

21 

18 

21 

18 

22 

19 

22 

19 

23 

20 

23 

20 

24 

21 

24 

21 

25 

22 

25 

22 

26 

23 

26 

23 

27 

24 

27 

24 

28 

25 

28 

25 

29 

26 

29 

26 

30 

27 

30 

27  • 

31 

28 

31 

28 

1 

29 

1 

29 

2 

30 

2 

30 

3 

31 

3 

31 

4 

FIG.  168. — The  wave  winding  of  Fig.  166  reconnected  for  a  120-volt  circuit 
by  connecting  two  coils  in  parallel  and  using  wider  brushes.  In  this  case  one' 
toil  is  dead  as  shown  by  the  heavy  lines. 


252 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


There  are  31  bars  in  the  original  commutator,  so  that  by  taking 
any  two  bars  and  putting  a  jumper  across  and  considering 
them  as  one  bar,  it  will  be  reduced  to  30  bars.  The  connections 
will  be  made  as  shown  by  the  following  table: 

TABLE  FOR  CONNECTING  COILS  OF  FIG.  168  TO  THE  COMMUTATOR 


Beginning  of  coil 
number 

To  bar  number 

End  of  coil  number 

To  bar  number 

Coil  1  is  not  con- 

nected. 

2 

28 

2 

12 

3 

29 

3 

13 

4 

30 

4 

14 

5 

1 

5 

15 

6 

2 

6 

16 

7 

3 

7 

17 

8 

4 

8 

18 

9 

5 

9 

19 

10 

6 

10 

20 

11 

7 

11 

21 

12 

8 

12 

22 

13 

9 

13 

23 

14 

10 

14 

24 

15 

11 

15 

25 

16 

12 

16 

26 

17 

13 

17 

27 

'    18 

14 

18 

28 

19 

15 

19 

29 

20 

16 

20 

30 

21 

17 

21 

1 

22 

18 

22 

2 

23 

19 

23 

3 

24 

20 

24 

4 

25 

21 

25 

5 

26 

27 

22 
23 

26 
'    27 

6 

7 

28 

24 

28 

8 

29 

25 

29 

9 

30 

26 

30 

10 

31 

27 

31 

11 

Reconnecting  Duplex  Windings.* — The  windings  discussed 
in   what  follows  are  wound  with  two  wires  in  hand.     The 
*  T.  Schutter,  Electrical  Engineering,  August,  1918. 


CHANGES  IN  DIRECT-CURRENT  MOTORS 


253 


same  results  can  be  accomplished  by  two  windings,  using  one 
strand  (two  simplex  windings)  at  a  time.  In  a  winding  of  the 
duplex  type  there  are  usually  twice  as  many  commutator 
bars  as  there  are  slots  and  each  of  the  two  wires  is  connected  to 
separate  bars.  The  brush,  however,  will  cover  at  least  1J/2 
to  2  commutator  bars,  as  shown  in  Fig.  169,  which  is  the  wind- 
ing and  connections  for  a  120-volt  lap-wound  armature. 


12        34        1  la     2  2a      33a 


Slot  No. 


Original  120  Volt  Connection 

FIG.  169. — Duplex  lap  winding  for  a  120-volt  armature. 

Each  coil  consists  of  two  parts,  which  will  be  called  Sec- 
tion 1,  and  la,  Section  2  and  2a,  Section  3  and  3a,  etc.  The 
following  is  the  winding  table  for  Fig.  169. 

TABLE  FOR  WINDING  COILS  IN  SLOTS  OF  FIG.  169 


Coil  number 

Coils  are  wound 

In  spaces  number 

In  slots  number 

1-la 

1  and  6 

1  and    3 

2-2a 

3  and  8 

2  and    4 

3-3a 

5  and  10 

3  and    5 

4-4a 

7  and  12 

4  and    6 

5-5a 

9  and  14 

5  and    7 

6-6a 

11  and  16 

6  and    8 

7-7a 

13  and  18 

7  and    9 

8-8a 

15  and  20 

8  and  10 

9-9a 

17  and  22 

9  and  11 

10-10a 

19  and  24 

10  and  12 

11-lla 

21  and  2 

11  and    1 

12-12a 

23  and  4 

12  and    2 

This  120-volt  winding  is  to  be  changed  so  that  it  can  be 
operated  on  a  240- volt  circuit.     As  shown  in  Fig.  169  Section 


254         ARMATURE  WINDING  AND  MOTOR  REPAIR 
TABLE  FOR  CONNECTING  COILS  TO  THE  COMMUTATOR  OP  FIG.  169 


Start  of  coil  number 

To  bar  number 

End  of  coil  number 

To  bar  number 

1 

3 

1 

5 

la 

4 

la 

6 

2 

5 

2 

7 

2a 

6 

2a 

8 

3 

7 

3 

9 

3a 

8 

3a 

10 

4 

9 

4 

11 

4a 

10 

4a 

12 

5 

11 

5 

13 

5a 

12 

5a 

14 

6 

13 

6 

15 

6a 

14 

6a 

16 

7 

15 

7 

17 

7a 

16 

7a 

18 

8 

17 

8 

19 

8a 

18 

8a 

20 

9 

19 

9 

21 

9a 

20 

9a 

22 

10 

21 

10 

23 

lOa 

22 

*10a 

24 

11 

23 

11 

1 

lla 

24 

lla 

2 

12 

1 

12 

3 

12a 

2 

12a 

4 

1  and  la  of  coil  1  are  connected  in  parallel  through  the  brush 
connections,  and  by  changing  the  connections  so  that  Sec- 


Slot  No. 


Reconnected  for  2iO  Volts 

FIG.  170. — The  120-volt  lap  winding  of  Fig.  169  reconnected  for  240  volts  by 
connecting  the  turns  of  each  coil  in  series  and  using  smaller  brushes. 

tions  1  and  la  will  be  in  series  instead  of  in  parallel,  it  will  then 
be  possible  to  operate  the  winding  on  a  240-volt  circuit.    The 


CHANGES  IN  DIRECT-CURRENT  MOTORS 


255 


winding  table  for  Fig.  170  will  be  the  same  as  for  Fig.  169 
but  the  connecting  table  will  be  as  follows: 

TABLE  FOR  CONNECTING  COILS  OF  FIG.  170  TO  COMMUTATOR 


Start  of  coil  number 

To  bar  number 

End  of  coil  number 

To  bar  number 

1 

1 

1 

2 

la 

2 

1 

3 

2 

3 

2 

4 

2a 

4 

2 

5 

3 

5 

3 

6 

3a 

6 

3a 

7 

4 

7 

4 

8 

4a 

8 

4a 

9 

5 

9 

5 

10 

5a 

10 

5a 

11 

6 

11 

6 

12 

6a 

12 

6a 

13 

7 

13 

7 

14 

7a 

14 

la 

15 

8 

15 

8 

16 

8a 

16 

8a 

17 

9 

17 

9 

18 

9a 

18 

9a 

19 

10 

19 

10 

20 

lOa 

20 

107 

21 

11 

21 

11 

22 

lla 

22 

lla 

23 

12 

23 

12 

24 

12a 

24 

12a 

1 

The  brushes  used  on  Fig.  170  should  only  be  as  wide  as  one 
commutator  bar.  By  tracing  the  current  through  the  winding 
in  Fig.  169  it  will  be  seen  that  there  are  three  coils  in  series 
in  each  of  the  paths  or  circuits  from  the  positive  brush  to  the 
negative  brush.  In  Fig.  169  there  are  six  coils  in  each  path  or 
circuit,  or  twice  as  many  as  before  the  reconnection. 

If  the  winding  in  Fig.  169  had  been  connected  to  a  12-bar 
commutator  instead  of  a  24-bar  commutator,  the  reconnecting 
from  120  volts  would  have  been  somewhat  different.  In 
Fig.  169  the  numerals  from  1  to  12,  which  are  placed  above 
and  between  each  two  bars,  will  give  an  idea  of  how  the  con- 
nections would  look.  For  instance,  Sections  1  and  la  of 


256         ARMATURE  WINDING  AND  MOTOR  REPAIR 

coil  No.  1  would  be  connected  across  bars  Nos.  2  and  3,  instead 
of  Section  1  to  bars  3  and  5,  and  la  to  bars  4  and  6. 

Then  to  reconnect  the  winding  from  120  volts  to  240  volts 
when  only  12  commutator  bars  are  used,  Sections  1  and  la 
of  Coil  No.  1  would  have  to  be  connected  in  series,  but  not 
through  commutator  connections  as  in  Fig.  170.  By  omitting 
the  connections  of  commutator  bars,  No.  2,  4,  6,  8  and  10, 
etc.,  and  simply  splicing  the  end  of  Section  1  to  the  beginning 
of  Section  la  of  coil  No.  1,  the  same  results  will  be  obtained 
by  using  only  12  commutator  bars. 

Fig.  171  is  a  " duplex  wave"  winding  for  operation  on  a 
120-volt  circuit.  This  winding  consists  of  13  coils  wound  with 
two  strands  of  wire  and  connected  to  26  commutators  bars. 
This  winding  could  also  be  connected  to  a  13-bar  commu- 
tator. The  winding  table  for  Fig.  171  is  as  follows,  each  coil 
being  considered  as  two  Sections,  1  and  la. 


TABLE  FOR  WINDING  COILS  IN  SLOTS  FOE  FIG.  171 


Coil  number 

Coils  are  wound 

In  spaces  number 

In  slots  number 

1  and    la 

1  and    6 

1  and    3 

2  and    2a 

3  and    8 

2  and    4 

3  and    3a 

5  and  10 

3  and    5 

4  and    4a 

7  and  12 

4  and    6 

5  and    5a 

9  and  14 

5  and    7 

6  and    6a 

11  and  16 

6  and    8 

7  and    7a 

13  and  18 

7  and    9 

8  and    8a 

15  and  20 

8  and  10 

9  and    9a 

17  and  22 

9  and  11 

10  and  10a 

19  and  24 

10  and  12 

11  and  lla 

21  and  26 

11  and  13 

12  and  12o 

23  and    2 

12  and    1 

13  and  13a 

25  and    4 

13  and    2 

The  winding  as  it  is  now  placed  and  connected  will  operate 
on  a  120-volt  circuit.  It  is  desired  to  change  it  so  that  it 
will  operate  on  a  240-volt  circuit.  There  are  two  methods 
by  which  this  can  be  accomplished.  The  first  is  shown  in 


CHANGES  IN  DIRECT-CURRENT  MOTORS  257 

TABLE  FOR  CONNECTING  COILS  OF  FIG.  171  TO  COMMUTATOR 


Start  of  coil  number 

To  bar  number 

End  of  coil  number 

To  bar  number 

1 

23 

1 

9 

la 

24 

la 

10 

2 

25 

2 

11 

2a 

26 

2a 

12 

3 

1 

3 

13 

3a 

2 

3a 

14 

4 

3 

4 

15 

4a 

4 

4a 

16 

5 

5 

5 

17 

5a 

6 

5a 

18 

6 

7 

6 

19 

6a 

8 

6a 

20 

7 

9 

7 

21 

la 

10 

7a 

22 

8 

11 

8 

23 

8a 

12 

8a 

24 

9 

13 

9 

25 

9a 

14 

9a 

26 

10 

15 

10 

1 

lOa 

16 

lOa 

2 

11 

17 

11 

3 

lla 

18 

lla 

4 

12 

19 

12 

5 

12a 

20 

12a 

6 

13 

21 

13 

7 

13a 

22 

13a 

8 

12         34       llo      22a      3  3a 


Original  120  Volt  Connection 

FIG.  171. — A  duplex  wave  winding  for  a  120- volt  armature. 

Fig.  172.  In  this  case  it  will  be  seen  that  Section  1  of  coil 
1  is  dropped,  that  is,  it  is  not  connected  to  the  commutator. 
This  may  be  done  so  that  a  wave  winding  is  possible.  In- 
stead of  connecting  the  different  sections  of  a  coil  in  parallel 

17 


258         ARMATURE  WINDING  AND  MOTOR  REPAIR 


through  the  commutator  and  brush  connections,  they  are 
now  connected  in  series  in  the  same  way.  The  connecting 
table  is  as  follows : 

TABLE  FOR  CONNECTING  COILS  OF  FIG.  172  TO  COMMUTATOR 


Start  of  coil  number 

To  bar  number 

End  of  coil  number 

To  bar  number 

la 

21 

la 

9 

2 

22 

2 

10 

2a 

23 

2a 

11 

3 

24 

3 

12 

3a 

25 

3a 

13 

4 

1 

4 

14 

4a 

2 

4a 

15 

5 

3 

5 

16 

5a 

4 

5a 

17 

6 

5 

6 

18 

6a 

6 

6a 

19 

7 

7 

7 

20 

7a 

8 

7a 

21 

8 

9 

8 

22 

8a 

10 

8a 

23 

9 

11 

9 

24 

9a 

12 

9a 

25 

10 

13 

10 

1 

lOa 

14 

lOa 

2 

11 

15 

11 

3 

lla 

16 

lla 

4 

12 

17 

12 

5 

12a 

18 

12a 

6 

13 

19 

13 

7 

13a 

20 

13a 

8 

It  will  be  seen  that  bars  Nos.  26  and  1  are  bridged  and  are 
acting  as  one  bar.  This  is  due  to  the  dropping  of  Section  1 
in  coil  No.  1  for  reasons  given  above.  By  tracing  the  flow  of 
the  current  from  the  positive  to  the  negative  brush  in  both 
Figs.  171  and  172,  is  will  be  found  that  there  are  twice  as 
many  coils  in  series  in  Fig.  172  as  there  are  in  Fig.  171. 
It  is,  therefore,  possible  to  operate  Fig.  172  on  a  240- volt 
circuit. 

The  other  method  of  reconnecting  Fig.  171  so  as  to  operate 


CHANGES  IN  DIRECT-CURRENT  MOTORS 


259 


it  on  a  240- volt  circuit  is  shown  in  Fig.  173.     The  coils  in 
this  case  have  been  placed  the  same  as  in  Figs.  171  and  172, 


1     la    2  2a   3  3 


FIG.   172. — The  duplex  wave  winding  of  Fig.  171  reconnected  for  240  volts  by 
connecting  the  coils  in  series  through  the  commutator  and  the  brushes. 


12      3* 


FIG.  173. — The  duplex  wave  winding  of  Fig.  171  reconnected  by  connecting 
the  turns  of  each  coil  in  series  and  using  one-half  the  number  of  commutator 
bars.  In  this  case  two  adjacent  bars  must  be  connected  together  and  brushes 
wide  enough  to  cover  two  bars. 

therefore  the  winding  table  will  be  the  same.     The  connect- 
ing table  is  as  follows: 

TABLE  FOR  CONNECTING  COILS  OP  FIG.  173  TO  COMMUTATOR 


Start  of  coil  number 

To  bar  number 

End  of  coil  number 

To  bar  number 

1 

•  23 

la 

9 

2 

25 

2a 

11 

3 

1 

3a 

13 

4 

3 

4a 

15 

5 

5 

5a 

17 

6 

7 

6a 

19 

7 

9 

7a 

21 

8 

11 

8a 

23 

9 

13 

9a 

25 

10 

,  •     15 

lOa 

1 

11 

17 

lla 

3 

12 

19 

12a 

5 

13 

21 

13a 

7 

260         ARMATURE  WINDING  AND  MOTOR  REPAIR 

From  the  above  connecting  table  it  will  be  seen  that  the 
end  of  Section  1  is  connected  to  the  beginning  of  Section  la 
or,  in  other  words,  the  two  sections  of  a  coil,  which  were  in 
parallel  with  one  another  in  Fig.  171  are  now  connected  in 
series. 

As  already  explained,  the  number  of  turns  per  coil  will  be 
directly  proportional  to  the  voltage.  In  Fig.  171  assume 
that  there  are  20  turns  per  coil  with  two  wires  in  parallel.  By 
reconnecting  as  in  Fig.  173,  there  will  be  40  turns  per  coil, 
using  one  wire.  From  this  it  will  be  seen  that  while  there  are 
13  coils  in  each  winding,  Figs.  171  and  173,  one  has  twice  as 
many  turns  per  coil  as  the  other.  This  fact  makes  it  pos- 
sible to  operate  Fig.  173  on  a  240-volt  circuit. 


CHAPTER  XI 

PRACTICAL  WAYS  FOR  RE- CONNECTING  INDUCTION 

MOTORS 

There  are  certain  changes  in  the  windings  of  an  induction 
motor  that  can  be  made  by  the  repairman  to  adapt  a  motor  to 
changed  operating  conditions.  There  are  also  certain  changes 
that  should  not  be  attempted.  Details  of  the  most  practical 
re-connections  and  their  effects  on  motor  operation  have  been 
carefully  outlined  accompanied  by  diagrams,  in  the  Electrical 
Journal  by  A.  M.  Dudley.  These  details  are  summarized  in 
what  follows. 

The  changes  that  can  be  made  in  the  connections  of  the 
induction  motor  may  be  divided  into  three  classes.  The  first 
class  includes  those  which  leave  the  motor  entirely  normal  and 
the  performance  in  all  essential  respects  the  same  as  before 
re-connection.  Such  changes,  for  example,  are  represented  by 
connecting  the  polar  groups  of  a  winding  in  series  for  440  volts 
and  in  parallel  for  220  volts.  The  second  class  of  changes 
leaves  the  performance  in  some  respects  unchanged  and  alters 
it  in  others.  These  may  be  represented  by  operating  a  motor 
in  star  on  440  volts  and  in  delta  for  220  volts.  In  this  case 
there  is  little  change  in  efficiency  or  power  factor;  the  starting 
and  maximum  torque,  however,  are  only  75  per  cent,  of  their 
original  values.  In  such  a  case  the  advisability  of  the  change 
depends  upon  the  work  that  the  motor  must  do.  If  the  torque 
at  the  altered  values  is  sufficient  to  start  and  carry  the  driven 
load  easily,  there  is  no  objection  to  operating  the  motor  indefi- 
nitely so  reconnected,  since  the  motor  will  not  run  any 
warmer  than  before  and  its  efficiency  and  power  factor  may  be 
better. 

The  third  class  of  changes  leaves  the  motor  operative  in  the 
sense  of  producing  torque  enough  to  do  the  work  required, 
but  so  alters  its  performance  as  to  heating,  or  efficiency  or 
power  factor  or  insulation  that  it  is  undesirable  to  leave  the 

261 


262         ARMATURE  WINDING  AND  MOTOR  REPAIR 

motor  operating  indefinitely  in  such  condition.  Such  changes 
are  represented  by  re-connecting  a  three-phase  motor  without 
changing  the  coils  for  two-phase  operation.  This  is  equiva- 
lent to  operating  the  three-phase  motor  at  125  per  cent,  normal 
voltage.  In  addition,  the  coils  which  should  have  extra  in- 
sulation where  the  phases  change  have  only  group  insulation. 
The  iron  loss  and  heating  may  be  increased  to  a  dangerous 
degree  and  the  power  factor  greatly  decreased.  Such  changes 
should  only  be  used  in  an  emergency  and  the  proper  permanent 
changes  made  as  soon  as  possible. 

Points  to  Consider  before  Making  Re -connections. — Before 
a  repairman  attempts  to  make  a  radical  change  in  the  connec- 
tions of  an  induction-motor  winding  he  should  consider  care- 
fully the  limitations  of  the  design  and  the  effects  of  the  changes 
The  following  points  will  serve  as  a  guide. 

1.  Changes  in  voltage  alone  are  the  easiest  and  can  usually  be  made. 

2.  Changes  in  number  of  phases  alone  can  rarely  be  made  satisfactorily 
and  are  usually  only  makeshifts. 

3.  Changes  in  number  of  poles  are  limited,  due  to  the  mechanical  form 
of  the  coils. 

4.  Changes  of  frequency  alone  or  in  combination  with  voltage  or  phase 
can  sometimes  be  made  if  changes  in  speed  are  not  objectionable. 

5.  Complicated  changes  should  not  be  attempted  except  by  persons 
of  some  experience  and  should  be  handled  with  caution. 

6.  If  the  peripheral  speed  of  the  rotor  (which  equals  rotor  diameter 
in  feet  X  3.14  X  rpm.)  exceeds  7000  feet  per  minute  on  any  proposed 
change,  the  maker  of  the  motor  should  be  consulted  before  making  the 
change. 

7.  In  case  of  any  doubt  on  any  point,  refer  to  the  manufacturer  of  the 
machine. 

Diagrams   for   Different   Changes   of   Connections. — Two 

kinds  of  diagrams  are  most  used  to  indicate  the  connections 
to  be  made  in  induction  motors.  These  are  shown  in  Figs. 
174  and  175.  The  diagram  of  Fig.  174  is  a  three-phase,  four- 
pole  winding  connected  in  star  on  a  36-slot  core.  It  represents 
the  coils  as  they  would  look  if  removed  from  the  machine  and 
laid  on  a  table  with  the  actual  connections  made.  The  dia- 
gram of  Fig.  175  is  a  conventional  sketch  for  the  winding  of 
Fig.  174  showing  the  polar  groups.  The  latter  diagram  is 
much  used, 


RE-CONNECTING  INDUCTION  MOTORS 


263 


In  the  winding  illustrated,  since  there  are  three  times  four  or 
12  polar  groups  and  36  coils,  there  are  three  coils  connected 
in  series  to  form  a  polar-phase-group.  It  should  be  borne  in 


Full  lines  represent  coils  in  tops  of  slots, 
broken  lines  coils  in  bottoms  of  slots 

FIG.   174. — Complete  winding  diagram  for  a  3-phase,  4-pole  motor  having 
36  slots,  and  connected  in  series-star. 

mind  that  there  are  not  actually  12  magnetic  poles  in  the 
machine,  for  the  reason  that  three  consecutive  polar-phase- 
groups  unite  to  form  orje  magnetic  pole  by  virtue  of  the  phase 
difference  of  the  currents  in  the  three  phases.  There  are  two 
magnetic  north  and  two  magnetic  south  poles  formed  by  this 
winding  at  any  instant,  and  these  poles  are  equally  spaced 
around  the  air-gap  like  four  mechanically  projecting  pole 
pieces  excited  by  four  coils  carrying  direct  current.  Some- 
thing of  this  conception  is  gained 
if  one  imagines  the  armature  of 
a  direct-current  generator  held 
stationary  and  the  field  poles 
rotated  around  it.  At  any  given 
instant  the  magnetic  field  can 
be  conceived  to  be  the  same  as 
the  field  which  is  formed  by 
the  winding  shown  in  Fig.  174. 
The  coils  in  slots  3,  6,  9,  12, 
etc.,  are  shown  by  heavy  lines 
to  indicate  that  the  insulation 

on     these     Coils     is      heavier     to   FlG-  175- — Circle  diagram  for  the 
•j.u  j    XT.  •  connections  shown  in  Fig.  174. 

withstand  the  greater  strain  at 

the  points  where  the  winding  crosses  or  lies  adjacent  to  coils 
differing  greatly  in  potential.  This  is  called  "phase  insula- 
tion," and  may  be  put  on  the  first  coil  in  each  group  or  it 


264         ARMATURE  WINDING  AND  MOTOR  REPAIR 

may  be  put  on  the  first  and  last  coil  of  each  group  where  there 
are  a  large  number  of  coils  in  the  group.  This  is  one  of  the 
reasons  why  a  machine  may  not  at  times  be  reconnected 
for  another  number  of  poles  or  phases.  If  such  reconnections 
were  made  the  maximum  differences  in  potential  might  occur 
between  two  adjacent  coils  unprotected  by  this  extra  insula- 
tion and  a  break-down  result. 

The  conventional  diagram  of  Fig.  175  represents  the  same 
connections  as  Fig.  174,  except  that  each  pole-phase-group 
as  numbered  1,  2,  3,  etc.,  in  Fig.  174,  is  shown  by  a  short  arc 
in  Fig.  175.  The  numbers  on  the  groups  are  identical  with 
Fig.  174,  and  the  group  connections.  The  arrows  are  shown 
simply  to  indicate  a  method  of  checking  up  to  insure  the  proper 
phase  relations.  There  is  considerable  danger,  in  a  three- 
phase  connection,  of  getting  a  60-degree  relation  between  the 
phases  instead  of  a  120-degree  relation  or,  as  it  might  be  ex- 
pressed on  the  diagram,  there  is  danger  that  the  wrong  end  of 
the  B  phase,  for  example,  may  be  connected  to  the  star  point. 
As  a  check  against  this,  when  the  diagram  is  completed,  the 
current  is  assumed  as  going  in  at  all  three  leads  toward  the 
Y  point.  Arrows  are  put  on  each  pole-phase-group  as  shown, 
and  when  all  three  phases  are  traced  through,  the  winding  is 
correct  if  the  arrows  on  consecutive  groups  run  alternately 
clockwise  and  counter-clockwise.  It  may  be  argued  that  this 
is  an  artificial  assumption  and  that  at  no  instant  is  the  current 
flowing  toward  the  star  in  all  three  phases.  It  may  also  be 
argued  that  in  a  correct  winding,  if  the  current  be  assumed  as 
flowing  toward  the  star  in  two  phases  and  always  from  it  in 
the  third,  the  arrows  will  fall  in  successive  sets  of  three  in  the 
same  direction  and  then  three  in  the  reverse  direction.  These 
statements  are  true,  but  a  little  experimenting  will  show  that 
an  incorrectly  connected  or  60-degree  winding  can  in  this  way 
be  shown  to  give  successive  sets  of  three  arrows  and  still  be 
wrong.  There  is  but  one  exception  to  the  correctness  of  the 
check  as  shown  in  Fig.  175  where  the  current  is  assumed  as 
flowing  toward  the  star  in  all  three  phases  and  the  arrows 
alternate  in  direction.  This  exception  to  the  rule  is  the  case 
where  the  winding  forms  consequent  poles  or  passes  through 
all  the  pole-phase-groups  in  a  north  direction  instead  of 


RE-CONNECTING  INDUCTION  MOTORS 


265 


alternately  north  and  south.     Such  connections  are  rarely  used , 
and  then  usually  on  special  motors  wound  for  multi-speeds. 
Diagrams  for  Three-phase  Motors. — Fig.  176  gives  a  com- 


B   C    A 


FIG.  176. — Diagram  for  a  3-phase, 
4%pole  winding  with  parallel-star  end 
connections.  Schematic  equivalent 
in  center. 


FIG.  177. — Diagram  for  a  3-phase, 
4-pole  winding  with  4-parallel-star 
end  connections.  Schematic  equiva- 
lent in  center. 


(A-BXB-CXA-CV 

Fio.  178. — Diagram  for  a  3- 
phase,  4-pole  winding  with  series- 
delta  end  connections.  Schematic 
equivalent  in  center. 


FIG.  179. — Connecting  diagram  for 
a  3-phase,  4-pole,  parallel-delta  wind- 
ing. A  schematic  equivalent  is  shown 
in  the  center  of  the  diagram. 


bined  conventional  and  schematic  representation  of  a  so-called 
parallel  star  diagram,  where  the  two  halves  of  each  phase  are 


266         ARMATURE  WINDING  AND  MOTOR  REPAIR 

\ 

in  parallel.  If  a  given  machine  were  connected,  as  shown  in 
Fig.  175,  for  a  normal  voltage  of  440,  it  could  readily  be  recon- 
nected according  to  Fig.  176,  and  would  then  be  suitable  for 
operation  on  220  volts  having  the  same  performance  in  all  re- 
spects except  that  it  would  draw  from  the  220-volt  line  twice 
as  many  amperes  under  a  given  load  as  it  previously  drew  from 
the  440-volt  line.  Similarly,  if  it  had  four  poles,  or  a  multiple 
of  four  poles,  it  could  still  be  paralleled  again,  or  put  in  4- 
parallel  star,  as  shown  in  Fig.  177,  and  operated  on  110  volts, 
and  would  still  have  the  same  performance  at  a  correspond- 
ingly increased  current  at  the  same  load.  ' 

Fig.  178  represents  a  variation  in  connection  from  the  fore- 
going, which  is  possible  only  with  three-phase  machines. 
This  is  the  so-called  delta  or  mesh  connection.  If  a  machine 
connected  as  in  Fig.  174  for  440  vofts  be  reconnected  as  in 
Fig.  178  it  would  be  suitable  for  operation  on  a  circuit  having 
a  voltage  of  440  -f-  1.73  or  254  volts. 


COMPARISON  OF  MOTOR  VOLTAGES  WITH  VARIOUS  CONNECTIONS 

If  a  motor  connected  originally  as  shown  in  any  horizontal  column  had 
a  normal  voltage  of  100  its  voltage  when  reconnected  as  indicated  in  any 
vertical  column  is  shown  at  the  intersection  of  the  two  columns. 


02 

QQ 

02 

O2 

1 

V 

Q 

« 
P 

° 

a 

13 

« 

1 

J 
1 

o3 
£ 
N 

1 
CO 

1 
4< 

1 

»o 

J 
1 

i 

e!, 

- 

CO 

- 
•* 

1 

U5 

1 

1 

t 

CO 

PH 
•^ 

1 

«o 

% 

fi 

co 

_c 
b 
co 

6 

£ 

& 

A 

£ 

CO 

£ 

CO 

x 

- 

CO 

£ 

CO 

£ 

CO 

£ 
et 

£ 

e!, 

fi 
<A 

4 
<N 

£ 
.A 

3-Phase  Series  Star  

« 
100 

50 

33 

25 

20 

58 

29 

19 

15 

12 

81 

41 

27 

20 

16 

3-Phase  2-Parallel  Star.  . 

200 

100 

67 

50 

40 

116 

58 

38 

29 

23 

162 

81 

54 

40 

32 

3-Phase  3-Parallel  Star.. 

300 

150 

100 

75 

60 

174 

87 

57 

44 

35 

243 

122 

81 

60 

48 

3-  Phase  4-Parallel  Star.  . 

400 

200 

133 

100 

80 

232 

116 

76 

58 

46 

324 

163 

108 

80 

64 

3-Phase  5-Parallel  Star.. 

500 

250 

165 

125 

100 

290 

145 

95 

73 

58 

405 

203 

135 

100 

80 

3-Phase  Series  Delta  

173 

86 

58 

43 

35 

100 

50 

33 

25 

20 

140 

70 

47 

35 

28 

3-Phase  2-Par.  Delta.  .  .  . 

346 

172 

116 

86 

70 

200 

100 

66 

50 

40 

280 

140 

94 

70 

56 

3-Phase  3-Par.  Delta.... 

519 

258 

174 

129 

105 

300 

150 

100 

75 

60 

420 

210 

141 

105 

84 

3-Phase  4-Par.  Delta.... 

692 

344 

232 

172 

140 

400 

200 

133 

100 

80 

560 

280 

188 

140 

112 

3-Phase  5-Par.  Delta  

865 

4-30 

290 

215 

175 

500 

250 

165 

125 

100 

700 

350 

235 

175 

140 

2-Phase  Series  

125 

63 

42 

31 

25 

73 

37 

24 

18 

15 

100 

50 

33 

25 

20 

2-  Phase  2-Parallels  

250 

125 

84 

63 

50 

146 

73 

49 

37 

29 

200 

100 

67 

50 

40 

2-Phase  3-Parallels  

375 

188 

125 

94 

75 

219 

110 

73 

55 

44 

300 

150 

100 

75 

60 

2-  Phase  4-Parallels  

500 

250 

167 

125 

100 

292 

146 

97 

73 

58 

400 

200 

133 

100 

80 

2-Phase  5-Parallels  

625 

313 

208 

156 

125 

365 

183 

122 

91 

73 

500 

250 

167 

125 

100 

RE-CONNECTING  INDUCTION  MOTORS 


267 


Use  of  Table  of  Connections. — Different  reconnections  or 
conversions  are  shown  in  the  Table  on  page  266  where  the 
problem  just  shown  may  be  worked  out  by  selecting  Three-phase 
Series  Star  in  the  horizontal 
column  (first  line)  and  reading 
across  to  the  vertical  column 
headed  Series  Delta  where  the 
figure  58  appears.  This  means 
that  if  100  volts  was  normal  on 
the  series  star  connection  and  a 
change  is  made  to  series  delta 
the  corresponding  voltage  is  58. 
By  multiplication,  if  440  was 
the  series  star  voltage,  the  series 
delta  voltage  would  be  4.4  X  58 
=  254,  as  noted  above.  Figs. 
179  and  180  show  a  parallel 
and  4-parallel  delta  connec- 
tion, respectively,  and  bear 
the  same  relation  to  Fig.  178  that  Figs.  176  and  177  do  to  Fig. 
175. 

Two-phase  Diagrams. — In  Fig.  181  a  development  of  a  two- 
phase  winding  is  shown  similar  to  Fig.  174,  except  for  the  differ- 
ence in  the  number  of  phases.  An  inspection  of  the  coils 
represented  in  heavy  lines  and  a  comparison  with  the  coils  in 


B  c  A 

(,B-C)(A-C)IA;B) 


FIG.  180.  —  Connecting  diagram 
for  a  3-phase,  4-pole,  4-parallel- 
delta  winding. 


Full  lines  represent  Coils  in  tops' of  slots,  broken  lines  coils  in  bottoms  of  slots 

FIG.  181. — Complete  winding  diagram  for  a  2-phase,   4-pole  motor  series 
connected.     The  coils  from  x  to  y  form  one  pole-phase-group. 

Fig.  174  indicates  at  once  what  is  meant  by  the  " phase  coils" 
or  phase  insulated  coils  being  differently  situated.  This  also 
explains  one  of  the  good  reasons  why  two-phase  motors  should 


268         ARMATURE  WINDING  AND  MOTOR  REPAIR 

not  be  reconnected  for  three-phase,  or  vice  versa,  without 
changing  the  position  of  these  "  phase  coils. 
Fig.  182  gives  the  conventional  and  schematic  equivalent  cf 


FIG.  182. — Connecting  diagram  for 
a  2-phase,  4-pole  winding  with  series 
connections. 


FIG.  183. — Connecting  diagram  for 
a  2-phase,  4-pole  winding  with  parallel 
connections. 


Fig.  181.  The  arrows  shown  in  the  three-phase  diagrams  are 
omitted  here,  for  the  .reason  that  the  two  phases  are  not  inter- 
connected, and  the  only  effect 
of  reversing  one  phase  is  to 
reverse  the  direction  of  rotation 
of  the  motor.  This  is  readily 
corrected  by  reversing  the  two 
leads  of  one  phase  at  the  motor 
terminals.  Figs.  183  and  184 
give  parallel  and  4-parallel,  two- 
phase  connections  and  bear  the 
same  relation  to  the  series  con- 
nection as  the  three-phase  star 
and  delta  diagrams.  From  these 

nl  n2  °l  °z 

FIG.  184.— Connecting  diagram  2-parallel  and  4-parallel  connec- 
for  a  2-phase,  4-pole  winding  with  tions  it  may  be  readily  seen  that 

4-parallel  connections.  . ,  ,  r         i        • 

where  the  number  of  poles  is  a 

multiple  of  three,  as  6,  12,  18,  etc.,  there  is  a  possible  analogous 
3-parallel  connection;  also  where  the  number  of  poles  is  a 
multiple  of  five,  such  as  10,  20,  etc.,  there  is  a  corresponding 


RE-CONNECTING  INDUCTION  MOTORS  269 

possible  5-parallel  diagram.  These  are  the  connections  which 
are  indicated  in  the  Table  on  page  266  as  "3-parallel"  and 
"  5-parallel." 

Fig.  185  shows  a  possible  three-phase  connection  which 
may  be  made  from  a  two-phase  winding  by  a  method  similar 
to  the  Scott  transformer  connection.  It  is  a  connection  which 
should  be  used  only  as  a  temporary  expedient  until  better 
arrangements  can  be  made. 


-Coils  removed 
,  from  circuit 

;7 


IBs 

FIG.  185. — The  T-connection  by  which  a  2-phase  motor  may  be  operated  on 
a  3-phase  circuit. 

Meaning  of  the  Term  Chord  Factor. — It  is  well  known  that 
the  span  of  the  coil  must  in  general  be  somewhere  near  the 
quotient  of  the  bore  periphery  divided  by  the  number  of  poles. 
It  is  not  so  generally  understood  that  changing  the  span  of  the 
coil  within  limits  has  an  effect  similar  to  increasing  or  decreas- 
ing the  number  of  wires  in  the  coil.  If  the  coil  is  exactly  pitch, 
i.e.,  spans  exactly  from  the  center  of  one  pole  to  the  center  of 
the  next,  the  turns  of  wire  in  that  coil  are  producing  their 
maximum  effect  upon  the  magnetic  field.  The  coil  is  then 
considered  to  span  180  electrical  degrees.  It  is  customary  to 
wind  the  coil  in  slots  so  that  it  spans  something  less  than  a 
full  pole  pitch.  The  effect  of  the  turns  in  the  coil  is  then  some- 
what less  than  the  maximum.  The  effect  of  the  turns  in  the 
coil  varies  as  the  sine  of  half  of  the  angle  in  electrical  degrees 
which  the  coil  spans. 

To  illustrate,  if  there  are  72  slots  in  an  eight-pole  machine, 
the  coils  would  be  exactly  " pitch"  if  they  lay  in  slots  1  and  10, 


270         ARMATURE  WINDING  AND  MOTOR  REPAIR 

or  in  other  words,  if  there  were  eight  slots  between  the  two 
slots  in  which  the  two  sides  of  any  coil  were  located.  Such  a 
coil  would  span  180  electrical  degrees.  Half  of  180  degrees  is 
90  degrees  and  the  sine  of  90  degrees  is  1 ;  therefore  the  effect 
of  the  turns  in  such  a  coil  is  one.  Suppose  instead  the  coil  lies 
in  slots  1  and  8.  It  would  then  t  span  140  degrees  elec- 
trically, since  72  -f-  8  =  9  slots  represents  180  degrees,  and 
one  slot  therefore  represents  20  degrees.  The  sine  of  half  of 
140  degrees,  or  70  degrees,  is  0.938.  It  follows  that  the  effect 
of  the  turns  in  this  coil  is  less  than  that  of  the  full  pitch  coil 
by  the  ratio  of  0.938  to  1.  This  is  of  interest  in  the  present 
problem,  because  it  is  often  possible  in  making  changes  to 
change  at  the  same  time  the  span  of  the  coils  by  one  slot, 
more  or  less,  by  springing  the  coil  mechanically,  and  so  im- 
prove the  performance  of  the  machine  under  the  new  con- 
ditions. The  point  becomes  of  vital  importance,  immediately, 
when  changing  the  number  of  poles  without  changing  the 
throw  of  the  coils.  Referring  again  to  the  72- slot  motor, 
assume  that  the  coils  are  wound  in  slots  1  and  8.  For 
an  eight-pole  connection  these  coils  will  have  the  effect  of 
0.938,  as  explained  above.  If  the  connections  are  changed 
for  six  poles  the  effect  is  entirely  different.  72  -r-  6  =  12  and 
180  -:-  12  =  15,  or  each  slot  represents  15  electrical  degrees. 
A  throw  of  1  and  8  covers  seven  complete  slots,  or 
7  X  15  =  105  degrees;  the  sine  of  half  of  105  or  52.5  degrees 
=  0.79,  which  means  that  when  connected  for  six  poles 
the  coils  have  an  effect  of  only  0.79,  as  against  0.938  when 
connected  for  eight  poles.  It  is  possible  to  avoid  using  the 
sine  of  half  the  angle  and  secure  a  factor  which  is  sufficiently 
accurate  practically  by  using  the  expression, 

(Number  of  slots  per  pole)2  —  2 (Number  of  slots  dropped)2 
(Number  of  slots  per  pole)2 

Using  the  same  eight-pole  example  above,  the  number  of 
slots  per  pole  is  72  -r-  8  =  9  and  the  pole  pitch  is  1  and  10. 
When  the  coil  is  wound  1  and  8  it  spans  seven  slots  and  there 
are  9  —  7=2  slots  dropped.  The  expression  then  becomes 


=  A  ^  =  0.948 


RE-CONNECTING  INDUCTION  MOTORS  271 

and  similarly  for  the  six-pole, 


(12)2  -  2(52) 
122 

which  agrees  roughly  with  the  other  method.  A  coil  should  in 
no  case  be  chorded  more  than  half  of  the  pole  pitch,  as  second- 
ary disturbances  of  the  magnetic  field  are  occasioned  by  chord- 
ing  which  become  prohibitive  at  that  point.  The  expression, 
"sine  of  half  the  angle  spanned  by  the  coil,"  is  given  the  name 
chord  factor,  and  it  should  be  considered  in  the  work  of  re-con- 
necting. For  example,  if  the  poles  are  changed  from  8  to  6, 
as  in  the  example  above,  and  the  chord  factor  changes  from 
0.938  to  0.79,  the  new  line  voltage  should  be  0.79  •*-  0.938 
times  the  old,  neglecting  the  effect  of  other  changes  which  are 
being  made.  If  nothing  else  was  undergoing  change  and  the 
-normal  voltage  was  440  in  the  first  place,  it  should  be  370  after 
the  change  is  made  or,  expressing  it  another  way,  if  it  was  still 
operated  at  440  volts  after  the  change,  the  motor  should  be 
thought  of  as  operating  at  about  18  per  cent,  over  voltage. 

Phase  Insulation. — It  is  the  practice  of  many  manufac- 
turers to  put  heavier  insulation  on  the  coils  at  the  ends  of  the 
polar  groups  which  are  mechanically  adjacent  to  one  another 
and  are  also  subjected  to  the  voltage  between  phases,  which 
may  be  the  maximum  voltage  between  supply  lines.  Such 
coils  are  illustrated  at  Nos.  3,  6,  9,  12,  15,  etc.,  in  Fig.  174. 
By  comparing  this  diagram  with  Fig.  181  for  two-phase 
connection,  it  appears  at  once  that  both  the  number 
and  location  of  these  so-called  "phase  coils"  should  be 
changed  at  the  time  the  machine  is  re-connected  from  two-  to 
three-phase,  or  vice  versa,  assuming  that  the  voltage  can  be 
changed  so  that  a  phase  change  is  permissible.  Also  in 
changing  the  number  of  poles,  the  number  and  location  of  the 
"phase  coils"  must  also  be  changed.  In  fact,  whatever  re- 
connection  is  attempted  the  "phase  coils"  should  be  checked 
and  re-arranged,  since  this  is  comparatively  easy  and  adds 
considerably  to  the  protection  of  the  machine  from  break- 
downs of  insulation. 


272         ARMATURE  WINDING  AND  MOTOR  REPAIR 

RE-CONNECTING  MOTORS  TO  MEET  NEW  OPERATING 
CONDITIONS 

Re-connections  Frequently  Made. — The  following  changes 
are  made  on  account  of  changes  in  operating  conditions  or  the 
service  conditions  on  the  circuits  from  which  the  motors  are 
operated. 

1.  Changes  to  operate  on  a  different  voltage. 

2.  Change  for  operation  on  a  different  phased  circuit,  three-  to  two- 
phase,  etc. 

3.  Changes  to  operate  on  a  different  frequency. 

4.  Change  in  number  of  poles  of  the  motor. 

In  case  of  change  No.  4,  the  re-connection  may  be  independent  of  all 
other  changes  to  secure  a  faster  or  slower  speed  or  it  may  follow  as  a  re- 
sult of  change  No.  3  in  order  to  keep  the  same  speed  on  a  driven 
machine  when  the  motor  is  operated  on  the  new  frequency. 

Procedure  when  Considering  a  Re-connection  of  Windings. 

The  procedure  in  checking  up  a  machine  to  see  whether  or 
not  it  can  be  re-connected  is  as  follows :  First,  ascertain  the  ex- 
isting connection  and  the  throw  of  the  coils  in  order  to  know 
what  the  possibilities  are  in  the  way  of  number  of  turns  and 
throw.  Second,  if  it  is  a  phase  or  voltage  change,  find  di- 
rectly from  the  Table  on  page  266  what  connections  will  give 
approximately  the  proper  new  voltage  and  new  phase.  If 
any  one  of  these  connections  is  possible  with  the  number  of 
poles  in  the  machine,  select  it  as  the  new  connection  and  ar- 
range the  phase  coils  properly  at  the  beginning  or  ending  of 
the  groups,  or  at  each  end  of  the  groups  if  there  are  enough  of 
them  in  the  old  winding.  Since  the  speed  has  not  changed,  the 
horsepower  should  remain  approximately  the  same,  and  the 
current  in  the  coils  themselves  will  remain  somewhere  near  the 
original.  If  the  frequency  is  to  be  changed  either  independ- 
ently or  in  conjunction  with  a  phase  or  a  voltage  change,  the 
applied  voltage  should  be  changed  in  the  same  direction  and 
by  the  same  proportional  amount  as  the  frequency  is  changed, 
or  if  the  voltage  is  to  remain  unchanged  the  number  of  turns  in 
series  in  the  coils  should  be  changed  in  the  opposite  direction 
to  the  frequency  and  by  the  same  amount.  For  example,  if  a 
25-cycle  motor  is  to  be  run  on  30  cycles,  it  should  have  the  volt- 
age increased  20  per  cent.,  or  else  have  the  groups  re-connected 


RE-CONNECTING  INDUCTION  MOTORS  273 

so  that  there  will  be  20  per  cent,  less  turns  in  series  and  run  on 
the  same  voltage. 

If  the  number  of  poles  is  to  be  changed,  and  consequently 
the  speed,  check  first  the  effect  of  the  coil  throw  or  chording 
with  the  new  number  of  poles.  Then  think  of  the  motor 
winding  as  generating  counter  emf.  and  bear  in  mind  that 
with  a  constant  field  a  higher  speed  will  generate  more  emf. 
and  a  slower  speed  less  emf.  Converted  into  voltage  this 
means  that  with  a  higher  speed  a  higher  voltage  should  be 
applied  in  direct  proportion  and  with  a  lower  speed  a  lower 
voltage  should  be  applied.  If  the  voltage  can  not  be  changed 
try  to  change  the  diagram  of  group  connections  so  as  to  vary 
the  number  of  turns  in  series  in  the  right  way,  that  is,  if  the 
voltage  should  be  increased,  the  same  effect  can  be  obtained 
by  decreasing  the  number  of  turns  a  like  amount.  In  all 
these  cases  it  is  the  voltage  per  turn  or  per  conductor  which 
counts,  just  as  in  a  transformer,  and  a  careful  consideration 
of  the  effect  of  different  connections  will  show  whether  the 
desired  change  in  voltage  per  conductor  is  being  accomplished. 

Practical  Example  for  Reconnection. — Assume  a  25-hp., 
four-pole  motor  operating  on  40  cycles,  two-phase,  220 
volts.  It  is  desired  to  know  whether  it  can  be  re-connected 
to  operate  on  60  cycles,  three-phase,  550  volts  at  the  same 
speed  and  horsepower.  An  inspection  of  the  machine  shows 
that  it  has  72  slots  and  72  coils  and  that  any  individual  coil 
lies  in  slots  1  and  15,  also  that  the  groups  are  connected  in 
parallel.  Since  there  are  72  -r-  4  =  18  slots  per  pole,  each 
slot  is  180  -r-  18  =  10  electrical  degrees  and  14  slots  =  140 
electrical  degrees.  (The  throw  of  1  to  15  means  spanning 
14  slots.)  The  sine  of  one-half  of  140  degrees  or  70  degrees 
=  0.94  =  chord  factor,  or  figured  by  the  formula  without  trig- 
onometry, since  there  are  18  slots  per  pole  and  a  throw  of 
1  to  15  means  dropping  four  slots  from  exact  pitch,  the  chord 

/182  —  2  X  42 
factor  =  -\l—       ^g2        -  =  0.948.      The  synchronous  speed 

of  the  motor  on  40  cycles  as  it  stands  is  4800  -r-  4  =  1200 
rpm.  To  get  this  same  speed  on  60  cycles  it  is  evident  the 
motor  will  have  to  be  connected  for  7200  -r-  1200  =  six  poles. 
If  the  throw  of  the  coils  be  left  1  to  15  they  will  throw  two  slots 

18 


274         ARMATURE  WINDING  AND  MOTOR  REPAIR 

further  than  full  pitch,  since  72  -f-  6  =  12  slots  per  pole  and 

1  to  13  would  be  exact  pitch.     Throwing  the  coil  over  pitch 
has  the  same  effect  as  throwing  it  under  pitch  so  the  new  chord 

/122 2  X  22 

factor  on  six  poles  =  A/—  22  -  =  0.97,  or  sine  of  one- 
half  of  150  degrees  =  0.98.  Taking  into  account  the  changes 
in  phase,  poles,  frequency  and  chording,  the  new  applied  volt- 
age per  phase  should  be  ~  X  |  X  ~  x  ^?  =  305  volts. 

o          o        40       0.94 

The  explanation  of  this  expression  by  terms  is:  The  first 
term,  (880  -r-  3)  comes  from  the  change  in  phase  from  2  to  3. 
Since  the  original  connection  was  in  parallel  and  was  for  two- 
phase,  the  voltage  across  one  phase  in  series  would  be  2  X  220 
=  440,  and  the  voltage  across  both  phases  in  series  would  be 

2  X  440  =  880  volts.     If  the  winding  is  divided  into  three 
separate  phases  not  interconnected,  the  applied  voltage  on 
each  phase  would  be  (880  -f-  3).     The  next  term,  (4  -r-  6)  is 
due  to  the  change  in  poles.     A  motor  with  six  poles  would  run 
slower  on  the  same  frequency  than  a  motor  with  four  poles  and 
would  generate  less  counter  emf.     Consequently,  the  applied 
voltage  should  be  decreased  in  the  same  proportion.     This 
should  not  be  confused  with  the  fact  that  the  frequency  is 
being  changed  in  this  case  and  the  speed  kept  the  same  be- 
cause a  separate  factor  is  introduced  to  take  care  of  the  fre- 
quency.    The  pole  change  should  be  considered  as  an  item 
separate  from  the  frequency  change.     The  next  term,  (60  -4- 
40)  is  due  to  the  change  in  frequency  and  is  the  application 
of  the  rule  to  change  the  applied  voltage  directly  as  the  fre- 
quency is  changed.     The  last  term,  (0.98  -f-  0.94)  is  due  to  the 
difference  in  chord  factor.     With  a  throw  of  1  to  15,  the  coils 
are  more  effective  to  generate  counter  emf.  on  the  six-pole 
than  on  the  four-pole  connection  by  the  ratio  of  the  chord 
factors  0.98  to  0.94,  hence  the  applied  voltage  should  be  raised 
with  the  counter  emf. 

The  value  of  305  volts  means  that  if  the  winding  was  divided 
into  three  separate  phases  not  interconnected  in  any  way,  the 
voltage  should  be  305  volts  across  each  phase.  If  the  three 
phases  are  connected  in  series  star>  as  in  Fig.  175,  the  applied 
voltage  should  be  1.73  X  305  =  530  volts.  Since  this  is  only 


RE-CONNECTING  INDUCTION  MOTORS  275 

about  3.5  per  cent,  off  from  the  550  volts  which  is  to  be  used, 
the  motor  will  operate  satisfactorily.  This  calculation  for 
voltage  so  far  neglects  the  difference  in  the  so-called  "  distri- 
bution factor"  between  three-phase  and  two-phase,  but  this  is 
immaterial.  This  factor  acts  the  same  way  as  the  chord 
factor,  and  is  about  0.955  for  any  normal  three-phase  windings 
and  0.905  for  any  normal  two-phase  winding,  so  that  the  ap- 
plied voltage  should  really  be  530  X  I  QQC)  =  5(30  volts, 

which  is  almost  exactly  what  is  required.  This  motor  could 
then  have  its  phase  coils  re-arranged  for  six  poles  and  be 
connected  series  star  and  would  be  satisfactory  for  the  new 
conditions.  The  changes  involved  do  not  materially  effect 
the  slip,  so  that  no  change  is  required  in  the  rotor  winding. 
This  example  illustrates  a  rough  calculation  that  can  be  made 
to  see  what  the  possibilities  of  re-connection  are. 

Changes  in  Voltage  Only  with  all  Other  Conditions  Re- 
maining the  Same. — This  is  the  simplest  change  which  can  be 
made  in  an  induction-motor  winding  and  in  principle  is  the 
same  as  that  of  a  transformer  coil  in  which  the  number  of  turns 
of  wire  in  series  must  be  varied  in  exact  proportion  to  the  vol- 
tage applied.  Practically  all  commercial  motors  are  arranged 
so  that  they  can  be  connected  for  two  voltages,  say  110  and 
220,  or  220  and  440.  This  is  accomplished  by  putting  the  polar 
groups  in  series,  as  in  Figs.  175  and  182,  for  the  higher  voltage, 
and  in  parallel,  as  in  Figs.  176  and  183,  for  the  lower  voltage. 

Assume  a  case  in  which  the  motor  as  it  stands  is  connected 
for  2200  volts  and  is  connected  in  series  star  as  in  Fig.  175. 
It  is  desired  to  re-connect  it  for  440  volts  for  the  same  horse- 
power, phase,  cycles  and  speed.  Four  hundred  and  forty 
volts  is  20  per  cent,  of  2200.  Refer  to  the  Table  on  page  266 
and  use  the  horizontal  column  marked  3-PAase,  Series  Star. 
Since  a  re-connection  is  desired  to  give  20  per  cent,  of  the 
original  voltage,  read  along  the  horizontal  line  until  the  figure 
20  occurs  This  is  found  first  under  the  vertical  column 
marked  3-Phase,  ^-Parallels.  This  is  obvious,  of  course, 
because  if  the  number  of  poles  in  the  machine  is  divisible  by 
five,  it  could  be  re-connected  in  five  parallels  and  operated  on 
2200  -T-  5  =  440  volts.  But  suppose  the  number  of  poles 


276         ARMATURE  WINDING  AND  MOTOR  REPAIR 

is  not  divisible  by  five.  Look  still  further  along  the  hori- 
zontal line  and  the  figure  19  appears  under  the  vertical 
column  headed  3-Phase,  ^-Parallel  Delta.  In  other  words,  if 
the  number  of  poles  in  the  machine  is  divisible  by  three  it  can 
be  put  in  three-parallel  delta  and  operated  on  2200  -r-  (3  X 
1.73)  =  424  volts,  which  is  near  enough  to  440  to  give 
perfectly  satisfactory  operation. 

If  the  number  of  poles  on  the  machine  is  not  divisible  by  five 
or  by  three,  it  is  evident  from  the  table  that  it  is  not  possible 
by  any  ordinary  three-phase  connection  to  approach  closer  than 
550  volts  with  a  four-parallel  star  connection,  or  330  volts  using 
a  four-parallel  delta  connection.  The  relation  between  550  volts 
and  330  volts  as  just  given  is  not  quite  the  theoretical  1.73 
which  would  be  expected,  but  this  is  due  to  the  table  being 
made  up  to  the  nearest  integral  figure  without  using  frac- 
tions. The  error  in  this  instance  is  three  per  cent,  which  is 
immaterial. 

A  point  which  is  brought  out  by  the  example  just  cited  is 
that,  so  far  as  insulation  is  concerned,  a  motor  may  always  be 
re-connected  for  a  lower  voltage — for  instance,  a  2200-volt 
motor  may  be  re-connected  for  440  volts — but,  on  the  contrary, 
a  motor  originally  designed  for  440  volts  may  not  be  run  on  2200, 
even  if  the  re-connection  is  possible  so  far  as  number  of  turns  is 
concerned,  because  the  insulation  will  not  stand  the  dielectric 
strain.  It  may  be  stated  generally  that  practically  all  manu- 
facturers use  two  classes  of  insulation  up  to  2500  volts,  one 
class  good  up  to  550  volts  and  the  second  good  from  600  volts  to 
2500.  This  should  be  carefully  considered  and  a  motor  never 
re-connected  from  the  lower  into  the  higher  class,  although 
the  change  from  the  higher  to  the  lower  is  permissible  from 
the  insulation  standpoint. 

Change  of  Phase  Only. — The  most  common  problem  which 
presents  itself  is  the  change  from  two-  to  three-phase,  and 
vice  versa.  Theoretically,  for  the  same  voltage  there  should 
be  about  25  per  cent,  more  total  turns  in  a  two-phase  winding 
than  in  a  three-phase  winding.  Then,  if  a  three-phase  motor 
be  re-connected  for  two-phase  at  the  same  voltage  and  with  the 
same  coils,  it  will  exhibit  all  the  symptoms  of  a  motor  operating 
at  25  per  cent  over  voltage  and  usually  would  overheat  to  a 


RE-CONNECTING  INDUCTION  MOTORS  277 

dangerous  degree  after  a  short  period  of  operation.  Con- 
versely, a  two-phase  motor  re-connected  and  run  on  three- 
phase  at  the  same  voltage  with  the  same  coils  will  show  all  the 
signs  of  a  motor  operating  at  20  per  cent,  under  voltage.  In 
this  case  there  are  too  many  turns  in  the  machine,  One-fifth 
of  the  total  coils  might  be  dead-ended  to  secure  the  proper  volt- 
age on  the  remaining  80  per  cent.  The  dead  coils  should  be 
distributed  as  symmetrically  as  possible  around  the  machine  to 
balance  the  voltage  as  nearly  as  possible  on  all  phases.  Parallels 
in  the  winding  should  be  avoided,  as  they  give  a  chance  for  un- 
balanced, circulating  local  currents,  which  may  cause  excessive 
temperatures.  Since  the  normal  full-load  current  on  a  three- 
phase  motor  at  any  given  voltage  is  about  12.5  per  cent,  greater 
than  the  two-phase  full-load  current  at  the  same  voltage,  it 
follows  that  the  three-phase  horsepower  will  have  to  be  cut 
down  about  12.5  per  cent,  from  the  two-phase  in  order  to  keep 
the  current  density  in  the  winding  as  it  was  on  two-phase. 
Unless  the  current  density  is  kept  approximately  the  same 
greater  heating  will  result.  Another  makeshift,  shown  in  Fig. 
185,  is  the  so-called  "Scott  or  T  connection  for  operating  a  two- 
phase  motor  on  three-phase.  By  this  scheme  14  per  cent,  of 
the  coils  in  one  phase  of  the  two-phase  machine  are  omitted  as 
symmetrically  as  possible  around  the  machine.  One  end,  BI 
of  this  phase,  is  then  connected  to  the  middle  of  phase  Ar~A2. 
The  resulting  voltages  between  the  points  Ai-Az-B^  are  prac- 
tically 4n  a  balanced  three-phase  relation.  This  connection 
would  give  fairly  good  results  if  the  coils  between  AI  and  BI 
were  so  situated  on  the  machine  that  they  would  be  acted 
upon  by  the  magnetic  field  in  exactly  the  same  manner  as 
the  coils  between  BI  and  A2.  Practically,  as  motors  are 
wound  nowadays,  this  is  rarely  possible,  and  if  the  usual  wind- 
ing is  connected  in  T  there  are  practically  always  unbalanced 
currents  in  the  three  phases.  The  current  in  the  high  phase 
will  be  about  20  per  cent,  greater  than  the  current  in  the  low 
phase.  This  results  in  a  poorer  performance  in  torque,  power- 
factor,  efficiency  and  heating.  The  efficiency  on  the  T  con- 
nection is  1.6  per  cent,  lower,  the  power-factor  5.2  per  cent, 
lower,  the  starting  torque  38  per  cent,  lower,  the  maximum 
torque  4  per  cent,  lower  and  the  temperatures  from  8  to 


278         ARMATURE  WINDING  AND  MOTOR  REPAIR 

13.5°  higher  than  on  the  normal  three-phase  winding.  This 
shows  that  changing  from  two-phase  to  three-phase,  and 
vice  versa,  is  at  best  very  unsatisfactory.  It  is  better  to  re- 
wind with  normal  three-phase  coils  and  avoid  the  troubles 
which  may  follow. 

One  essential  in  any  phase  re-connection  is  to  go  over  the 
winding  and  re-arrange  the  "  phase  coils/ '  or  coils  having 
heavier  insulation,  so  that  they  will  come  properly  at  the  ends 
of  the  groups  where  the  voltage  is  highest.  This  is  illustrated 
in  Figs.  174  and  181. 

One  case  of  voltage  and  phase  change  which  works  out  very 
well  is  the  change  from  three-phase  550  volts  to  two-phase 
440  volts,  or  vice  versa.  This  uses  all  the  turns  in  the  winding 
for  either  connection,  since  the  two-phase  voltage  should  be 
about  80  per  cent,  of  the  three-phase,  and  since  the  higher  volt- 
age on  the  three-phase  cuts  down  the  current,  which  would 
otherwise  be  higher  than  the  two-phase  circuit.  If  the  phase 
coils  are  re-arranged  there  is  practically  no  objection  to  such  a 
re-connection  and  the  motor  will  give  essentially  the  same  per- 
formance on  either  connection. 

The  table  on  page  266  shows  the  possibilities  of  interphase 
connections,  as  well  as  the  different  voltage  changes.  For 
example,  in  the  case  just  cited,  follow  the  horizontal  line 
marked  2-Phase  Series  to  the  first  vertical  column  headed  3- 
Phase  Series.  The  figure  is  125.  This  means  that  a  motor 
originally  connected  two-phase  series,  if  re-connected  three- 
phase  series,  should  be  operated  on  125  per  cent,  of  the  original 
voltage.  Or,  if  the  two-phase  voltage  was  440  the  three-phase 
would  be  1.25  X  440  =  550  volts.  The  convenience  of  the 
table  is  demonstrated  for  phase  changes,  as  well  as  voltage 
changes,  or  for  combinations  of  both. 

Changes  in  Frequency. — The  occasion  often  arises  for  chang- 
ing 25-cycle  motors  to  60-cycle  and  60  to  25.  There  is  also 
some  changing  done  from  60  cycles  to  50  and  50  to  60.  Occa- 
sionally 40-cycle  motors  are  changed  to  60,  but  these  changes 
are  infrequent. 

In  all  cases  of  changed  frequency  the  question  that  first 
arises  is:  How  is  the  resulting  change  in  speed  to  be  taken 
care  of?  The  synchronous  speed  of  any  motor  (which  is  only 


RE-CONNECTING  INDUCTION  MOTORS  279 

a  few  per  cent,  higher  than  the  full-load  speed)  is  given  by  the 

Alternations  per  Minute  . 

general  expression  -      Number  of  Poles •     This  would  be 

3000  ,     oe  7200  , 

XT r — -vm —  f°r  25  cycles,  ^ u £  r>  ^      f°r  60  cycles, 

Number  of  Poles  '  Number  of  Poles 

etc.  If  then  the  frequency  is  changed  and  the  number  of 
poles  left  the  same,  the  resulting  rpm.  will  vary  directly  as 
the  frequency.  This  immediately  brings  up  two  questions: 
First,  is  the  mechanical  design  of  the  rotating  part  adequate 
to  allow  such  a  change  in  speed?  Second,  can  the  speed  of 
the  driven  machine  be  adjusted  to  suit  the  new  speed  on  the 
motor? 

Consider  first  the  case  where  the  frequency  is  changed  and 
the  number  of  poles  remain  the  same.  The  resulting  change 
in  speed  in  this  case  is  taken  care  of  either  by  applying  the 
motor  to  a  new  load  or  by  changing  the  pulleys  on  the  old  load 
so  as  to  keep  the  same  rpm.  on  the  driven  machine.  The 
next  thing  that  must  be  considered  is  the  necessary  change 
in  the  voltage  applied  to  correspond  to  the  change  in  frequency, 
or  the  other  way  about,  if  the  new  circuit  at  the  new  frequency 
has  the  same  voltage  as  was  used  with  the  original  frequency, 
how  can  the  coils  in  the  motor  be  re-connected  so  as  to  get  the 
proper  voltage  on  each  coil? 

The  easiest  rule  to  remember  is  to  vary  the  applied  voltage 
on  the  motor  in  exactly  the  same  way  as  the  frequency  is  varied. 
If  this  be  done  the  magnetic  field  in  t*he  iron  will  remain  the 
same  and  the  current  in  the  stator  and  rotor  coils  will  remain 
the  same,  if  the  motor  is  working  against  the  same  torque. 
This  is  another  way  of  saying  that  if  the  frequency  and  voltage 
are  varied  together,  the  motor  will  develop  the  same  torque  at 
all  times  and  have  flowing  in  it  approximately  the  same  cur- 
rent. If  the  torque  remains  the  same,  the  horsepower  de- 
veloped will  vary  directly  as  the  applied  frequency.  For 
example,  a  60-cycle,  50-hp.  motor  operated  on  25  cycles  at 
41.6  per  cent,  of  its  original  voltage  would  develop  the  same 
normal  full-load  torque,  which  would  be  20.8  hp. 

Changing  from  25  to  60  cycles. — A  change  from  25  cycles  to 
60  cycles,  can  often  be  made  by  impressing  twice  the  voltage 
on  the  coils  on  60  cycles  as  on  25  cycles.  A  220-volt,  25-cycle 


280         ARMATURE  WINDING  AND  MOTOR  REPAIR 

motor  operated  on  440  volts,  60  cycles,  will  have  about  double 
the  horsepower.  Theoretically,  this  should  be  60  -r-  25  =  2.4 
times  the  voltage,  instead  of  twice,  and  the  resulting  horse- 
power would  be  2.4  times.  In  this  case  suppose  the  motor 
was  connected  in  series  star  for  440  volts  on  25  cycles  and  it  is 
desired  to  run  it  on  440  volts,  60  cycles.  It  should  then  be 
connected  in  parallel  star  and  run  on  440  volts,  which  would 
have  the  same  effect  as  impressing  880  volts  on  the  original 
series  connection.  On  60  cycles  the  motor  would  then  run 
2.4  times  as  fast  and  develop  about  twice  the  horsepower. 

Sixty-cycle  Motors  on  5Q-cycle  Circuits. — Sixty-cycle  motors 
are  often  run  on  50  cycles  without  change.  From  the  rule 
above,  that  the  voltage  must  vary  with  the  frequency  to  keep 
the  same  magnetic  densities,  it  will  be  noted  that  the  densi- 
ties on  50  cycles  at  the  same  voltage  will  be  six-fifths  of  the 
60  cycles  densities.  The  motor  will  then  operate  as  if  it  had 
120  per  cent,  of  normal  voltage  impressed.  This  will  result  in 
increased  iron  losses,  which  makes  the  motor  hotter,  and  the 
decreased  speed  on  50  cycles  with  the  same  number  of  poles 
also  makes  the  ventilation  poorer,  so  that  the  output  of  the 
motor  in  horsepower  should  be  reduced  to  keep  down  the  cop- 
per losses. 

Another  point  that  should  be  watched  in  changing  frequency 
if  the  motor  has  a  squirrel-cage  rotor,  is  to  make  sure  that  the 
rotor  winding  has  enough  resistane  to  give  the  proper  starting 
torque.  As  the  frequency  is  raised  the  resistance  of  the  short- 
circuiting  rings  at  the  ends  of  the  rotor  winding  should 
be  increased  to  keep  the  same  relative  value  of  starting  torque 
to  full-load  torque.  As  long  as  the  motor  starts  its  load  satis- 
factorily no  change  is  necessary,  but  if  trouble  is  experienced, 
the  short-circuiting  rings  may  have  to  be  changed  for  ones  of 
higher  resistance.  Conversely,  when  decreasing  the  frequency 
the  resistance  can  be  reduced  to  advantage,  thereby  cutting 
down  the  rotor  copper  loss  and  the  heating. 

Change  in  Frequency  with  Same  Speed. — In  this  case  the 
number  of  poles  must  be  changed  in  the  same  ratio  as  the 
frequency,  or  as  nearly  so  as  possible.  For  example,  if  a 
motor  has  four  poles  and  is  operated  on  25  cycles,  it  will 
have  a  synchronous  speed  of  3000  -f-  4  =  750  rpm.  If  the 


RE-CONNECTING  INDUCTION  MOTORS  281 

motor  is  to  have  the  same  speed  on  60  cycles,  the  nearest 
possible  pole  number  is  10  and  the  synchronous  speed  will  be 
7200  -r-  10  =  720  rpm.  It  is  apparent  that  in  very  few  cases 
of  this  kind  is  it  possible  to  re-connect  the  same  winding. 
The  main  reason  for  this  is  in  the  throw  or  pitch  of  the  coil. 
In  the  four-pole  winding  the  individual  coil  spans  approximate 
one-fourth  of  the  stator  bore,  and  in  the  10-pole  winding 
normal  coils  should  span  about  one-tenth  of  the  stator  bore. 
In  the  paragraph  on  "chorded  windings"  (page  269)  it  was 
pointed  out  that  the  coil  throw  has  an  effect  on  the  generated 
counter  emf  proportional  to  the  sine  of  one-half  the  electrical 
angle  spanned  by  the  coil.  This  consideration  makes  hardly 
possible  such  a  condition  as  connecting  a  winding  for  10  poles 
when  the  individual  coils  have  a  four-pole  throw.  When 
reducing  the  frequency  the  number  of  poles  should  become 
smaller  to  keep  the  same  speed.  This  introduces  another 
difficulty  in  the  magnetic  circuit.  In  re-connecting  the  wind- 
ing the  object  is  to  keep  the  total  magnetic  flux  in  the  machine 
the  same  as  it  was  originally.  This  keeps  the  magnetic 
density  in  the  teeth  constant.  This  total  magnetic  flux  is 
divided  up  into  as  many  equal  parts  or  circuits  as  there  are 
poles  The  iron  in  the  stator  core  between  the  bottoms  of  the 
slots  and  the  outside  of  the  core  has  to  carry  the  flux  for  each 
magnetic  circuit.  Consequently,  if  there  are  10  poles  and 
10  magnetic  circuits,  the  core  iron  below  the  slots  has  to  carry 
at  a  given  cross-section  one-tenth  of  the  total  magnetic  flux. 
With  the  same  total  magnetic  flux,  if  there  arc  only  four  poles 
and  four  magnetic  circuits,  the  same  cross-section  of  core  has  to 
carry  one-fourth  of  the  total  magnetic  flux,  which  it  is  probably 
unable  to  do.  This  is  the  reason  why  the  rotor  diameter  and 
stator  of  a  25-cycle  machine  are  smaller  than  those  of  a  60- 
cycle  machine  of  the  same  horsepower  and  speed,  although  the 
outside  diameter  may  be  nearly  the  same.  It  is  to  get  a 
larger  cross-section  behind  the  slots  for  the  passage  of  the 
magnetic  flux,  since  the  total  flux  is  divided  into  fewer  parts, 
owing  to  the  smaller  number  of  poles.  From  this  it  follows 
that  a  machine  may  in  general  be  re-wound  or  re-connected  for 
a  larger  number  of  poles,  but  that  great  caution  is  required  in 
re-connecting  for  a  smaller  number  of  poles. 


282         ARMATURE  WINDING  AND  MOTOR  REPAIR 

It  is  easier  to  re- wind  or  re-connect  25-cycle  machines  for  60 
cycles  than  it  is  to  re-connect  60-cycle  machines  for  25  cycles. 
This  follows  logically  from  the  physical  fact  that  there  is  more 
copper  and  more  iron  in  25-cycle  machines  for  the  same  horse- 
power, voltage  and  rpm.  than  in  60-cycle  machines.  It  is 
always  easier  to  make  changes  where  there  is  a  larger  supply 
of  material  available.  Another  condition  that  is  against  chang- 
ing the  number  of  poles  on  a  squirrel-cage  motor  is  the  cur- 
rent in  the  short-circuiting  rings  of  the  rotor  winding.  These 
rings  are  in  nearly  the  same  relation  as  regards  current  that 
the  primary  core  is  as  regards  magnetic  flux.  That  is,  the 
total  secondary  amperes,  which  remain  nearly  the  same  if 
the  re-connection  is  done  properly,  are  divided  into  as  many  cir- 
cuits as  there  are  poles,  and  it  follows  at  once  that  the  smaller 
the  number  of  poles  the  larger  must  be  the  cross-section  of  the 
short-circuiting  rings,  although  the  total  secondary  amperes 
remain  nearly  the  same.  Altogether,  the  possibility  of  re-con- 
necting for  different  numbers  of  poles  when  changing  frequency 
is  usually  a  matter  for  the  designing  engineer  to  investigate. 

Changes  in  the  Number  of  Poles,  all  Other  Conditions 
Remaining  the  Same. — The  need  for  such  changes  comes 
from  the  desire  to  speed  up  or  slow  down  the  driven  machine 
to  meet  new  requirements.  It  might  be  broadly  stated  that 
there  are  many  cases  where  a  change  of  two  poles  is  permis- 
sible, as  for  example,  changing  from  four  poles  to  six,  or  from 
ten  to  eight  and  the  like.  The  changes  would  consist  in  re- 
arranging the  phase  coils  to  agree  with  the  new  grouping  and 
checking  the  chord  factor,  to  note  its  effect  on  the  voltage. 
It  is  often  possible  to  get  a  fair  operating  half  speed  by  con- 
necting for  twice  the  number  of  poles.  Practically  all  re- 
connections  involving  pole  changes  give  only  a  fair  operating 
performance.  <^- 

Testing  a  Re-connected  Motor. — After  a  motor  has  been  re- 
connected or  after  any  change  is  made  in  the  winding,  it  should 
be  started  up  slowly  and  the  load  gradually  thrown  on,  ob- 
serving carefully  to  see  if  there  are  any  signs  of  distress,  such  as 
sudden  heating,  noise  or  mechanical  vibration.  If  the  motor 
seems  to  operate  normally  read  the  amperes  in  each  phase 
and  the  voltage  across  each  phase  to  see  that  they  are  balanced 


RE-CONNECTING  INDUCTION  MOTORS  283 

and  are  reasonable  in  amount.  The  full-load  current  for  three- 
phase  550  volts  is  somewhere  near  one  ampere  per  horse- 
power for  normal  motors  of  moderate  speeds  between  five  and 
200  horsepower.  At  other  voltages  this  will  be  inversely  as 
the  voltage,  that  is  at  440  volts,  three-phase,  about  1.25  am- 
peres per  horsepower.  On  two-phase  the  current  per  phase 
is  about  87  per  cent,  of  the  corresponding  three-phase  value. 
If  the  readings  as  above  look  reasonable  a  thermometer  should 
be  placed  on  the  stator  iron  and  another  on  the  stator  coils  and 
read  at  15-minute  intervals  for  an  hour,  and  at  half -hour  in- 
tervals thereafter,  until  the  temperature  is  constant.  The 
speed  should  be  checked  at  intervals.  If  the  rpm.  shows  a 
tendency  to  decrease  rapidly  or  fall  below  90  per  cent,  of  syn- 
chronous speed,  it  may  be  suspected  that  the  rotor  has  too 
much  resistance  and  is  getting  hot.  By  making  all  these 
checks,  reasonable  assurance  may  be  had  that  the  reconnec- 
tion  is  satisfactory. 

Effects  of  High  and  Low  Voltage  on  Motor  Operation. — 
All  changes  in  alternating-current  motors  whether  of  phase, 
voltage,  poles  or  frequency,  may  be  considered  as  voltage 
changes  and  reduced  to  such  terms.  In  making  such  calcula- 
tions and  comparing  the  results,  it  is  advisable  not  to  apply  a 
voltage  that  differs  from  the  rated  voltage  by  more  than  plus 
or  minus  10  per  cent.  The  general  effect  of  high  and  low 
voltage  may  be  expressed  briefly  as  follows: 

Effect  of  High  Voltage: 
a.  Increases  magnetic  density. 
6.  Increases  magnetizing  current. 

c.  Decreases  "leakage  current"  (leakage  reactive  component). 

d.  Increases  starting  torque  and  maximum  torque. 

e.  Decreases  slip  or  change  in  speed  from  no  load  to  full  load. 
/.  Decreases  secondary  copper  loss. 

g.  Increases  iron  loss. 

h.  Usually  decreases  power-factor. 

i.  May  increase  or  decrease  efficiency  and  heating,  depending  upon 
the  proportions  of  primary  copper  loss  and  iron  loss  in  the  normal 
machine  and  also  the  degree  of  saturation  in  the  iron. 

Effect  of  Low  Voltage: 

a.  Decreases  magnetic  density. 

b.  Decreases  magnetizing  current. 


284         ARMATURE  WINDING  AND  MOTOR  REPAIR 

c.  Increases  leakage  current. 

d.  Decreases  starting  and  maximum  torque. 

e.  Increases  slip. 

/.  Increases  secondary  copper  loss. 

g.  Decreases  iron  loss. 

h.  Usually  increases  power-factor. 

t.  May  increase  or  decrease  efficiency  and  heating,  depending  upon 
the  proportions  of  primary  copper  loss  and  iron  loss  in  the  normal 
machine  and  also  the  degree  of  saturation  in  the  iron. 

Operating  Standard  Alternating-current  Motors  on  Differ- 
ent Voltages  and  Frequencies. — When  a  motor  is  to  be  oper- 
ated on  a  different  voltage  or  frequency  than  the  motor  was 
designed  for,  there  should  be  a  corresponding  change  made  in 
circuit  to  which  the  motor  will  be  connected.  That  is,  if  there 
is  to  be  an  increase  in  frequency  or  voltage,  an  equal  decrease 
in  voltage  or  frequency  of  the  circuit  will  bring  about  normal 
results.  Present-day  designs  of  motors  can  be  used  in  most 
cases  without  excessive  heating  when  the  variation  either  up 
or  down  of  frequency  or  voltage  is  not  more  than  10  per  cent. 

For  an  induction  motor  the  following  conditions  result  with 
a  change  of  frequency  or  voltage: 

1.  Pull-out  torque  and  starting  torque  vary  as  the  square 
of  the  voltage  and  inversely  as  the  square  of  the  frequency. 

2.  The  copper  loss  in  the  primary  varies  as  the  square  of 
the  current.     The  current  varies  inversely  as  the  voltage,  but 
is  not  affected  by  a  change  in  frequency,  except  to  the  slight 
extent  produced  by  changes  in  magnetizing  current.     The 
secondary   copper   loss   and  slip   tend   to   vary  inversely  as 
the  square  of  the  voltage,  but  this  tendency  is  modified  by 
the  changes  in  primary  IR  drop  and  magnetic  leakage.     The 
secondary  copper  loss  and  slip  remain  constant  with  change 
in  frequency. 

3.  The  iron  loss  is  composed  of  hysteresis  and  eddy  current 
losses.     The  hysteresis  loss  varies  as  the  1.6  power  of  the  flux; 
the  eddy  current  loss  varies  with  the  square  of  the  flux;  and 
the  flux  varies  directly  as  the  voltage  and  inversely  with  the 
frequency.     The    magnetizing    current   varies   directly   with 
the  flux  except  for  modifications  produced  by  saturation  of  the 
magnetic  circuit. 

4.  The  power-factor  is  usually  decreased  by  an  increase  in 


RE-CONNECTING  INDUCTION  MOTORS 


285 


voltage  or  a  decrease  in  frequency  and  vice  versa,  but  the  total 
change  is  small. 

5.  The  efficiency  is  not  materially  altered  by  a  change  in 
either  frequency  or  voltage. 

The  accompanying  curve,  Fig.  186  shows  the  operating 
voltage  on  which  a  standard  motor  can  be  used  when  con- 


450 
400 

ocn 

X 

X 

>x 

X 

X 

300 
250 
200 
150 
100 
50 

x 

x' 

-X 

X 

X 

X 

/ 

/I 

]/ 

/ 

5      10      15     20     25      30     35     40     45     50     55     60      65     70 
Frequency     Cycles  per  Sec. 

FIG.  186. — Standard  motor  frequency  and  voltage  curve. 

This  curve  indicates  the  voltage  which,  if  employed  in  connection  with  the  respective 
corresponding  frequencies,  will  result  in  the  operation  of  apparatus  at  approximately 
uniform  core  densities.  By  adherence  to  the  relations  betweeen  frequency  and  voltage 
indicated,  the  range  of  application  of  standard  apparatus  can  be  broadened  and  the 
required  number  of  different  designs  minimized.  Allowance  is  made  for  the  use  of 
somewhat  reduced  densities  at  the  higher  frequencies,  as  indicated  by  the  drooping 
ciiaracter  of  the  curve  (R.  E.  Hellmund,  Electric  Journal,  September,  1910,  page  691). 

nected  to  a  circuit  of  a  different  frequency  from  that  for  which 
it  was  designed.  The  accompanying  table  also  shows  the 
effect  on  speed  and  horse-power  rating  when  a  motor  is  used 
on  a  different  voltage  and  frequency. 

The  voltage  and  frequency  of  a  motor  should  never  be  varied 
in  opposite  directions  at  the  same  time.  In  general  any  change 
from  normal  frequency  should  be  accompanied  by  a  change  in 
voltage  proportional  to  the  square  root  of  the  frequency.  In 
the  case  of  400-volt,  60-cycle  motor  operated  on  a  66%  cycle 
circuit,  the  voltage  should  be  422.  That  is  V(66%  -5-  60) 
X  400  =  422.  In  case  of  decreased  frequency,  the  motor 
should  be  operated  on  less  than  normal  voltage  on  account  of 
the  increased  current  and  temperature. 


286         ARMATURE  WINDING  AND  MOTOR  REPAIR 


EFFECT  ON  SPEED  AND  HORSEPOWER  WHEN  MOTORS   ARE  OPERATED 
ON  DIFFERENT  VOLTAGES  AND  FREQUENCIES 


Rating  of  motor 

Voltage  and  fre- 
quency of  circuit 

Speed 

Hp.  rating 

220  volts,  25  cycles 
440  volts,  25  cycles 
(a)   440  volts,  60  cycles 
440  volts,  60  cycles 
(6)   220  volts,  60  cycles 

250  volts,  33  cycles 
500  volts,  33  cycles 
220  volts,  25  cycles 
220  volts,  33  cycles 
220  volts,  50  cycles 

Increased  33   to   25 
Increased  33  to    25 
Reduced    60    to    25 
Reduced    60    to    33 
Reduced    60   to    50 

Increased  33  to    25 
Increased  33    to   25 
Reduced    60    to  '25 
Reduced    60    to    33 
Reduced    60   to    50 

(a)  Where  good  power-factors  are  essential  it  may  be  advisable  to  use  550-volt,  60- 
cycle  motor  or  a  220-volt,  25-cycle  circuit  and  reduce  the  rating  according  to  the  heating 
between  35  to  45  per  cent,  of  the  rating  at  60  cycles. 

(6)  Standard  60-cycle  motors  of  liberal  rating  can  be  used  on  a  50-cycle  circuit  with 
the  same  rating.  Best  results  are  secured  however,  when  220-,  440-,  and  550-volt 
motors  are  operated  on  200-,  400-  and  500-volt  circuits  at  50  cycles. 

Factors  which  Limit  a  Change  in  Number  of  Poles  of  an 
Induction  Motor. — The  principal  factors  limiting  a  change  in 
the  number  of  poles  of  a  squirrel-cage  induction  motor  are 
given  as  follows  by  a  writer  in  the  General  Electric  Review: 

(a)  The  number  of  turns  in  series  per  phase.  These  must  remain  the 
same  since  the  applied  voltage  is  to  be  unchanged. 

(6)  The  insulation  between  the  conductors  of  different  phases.  Of 
this  there  must  be  sufficient  to  not  reduce  the  factor  of  safety  against 
breakdowns  after  the  regrouping  of  the  conductors  has  been  carried  out. 

(c)  The  saturation  of  the  iron.  It  is  often  inadvisable  to  use  a  mag- 
netic density  much  higher  than  normal. 

Because  the  designs  of  induction  motors  vary  widely  with 
different  manufacturers  and  also  in  the  product  of  each  maker 
(for  the  purpose  of  supplying  motors  for  various  types  of 
service),  it  will  be  impossible  to  make  other  than  very  general 
statements  regarding  the  expected  change  in  characteristics 
of  the  motor  when  running  at  the  higher  speed.  Furthermore, 
the  following  statements  must  not  be  expected  to  hold  true 
when  the  number  of  poles  has  been  decreased  sufficiently  to 
raise  the  normal  speed  more  than  say  25  per  cent. 

After  the  reconnection, 

(a)  The  normal  speed  will  be  equal  to  approximately  the  original 
normal  speed  times  the  original  number  of  poles  divided  by  the  new 
number  of  poles. 

(6)  There  will  be  a  somewhat  higher  torque  per  pole  exerted,  due  to 
the  slightly  increased  flux  per  pole  that  arises  from  the  shortened  pole 
pitch,  so  that  the  total  motor  torque  might  be  expected  to  be  decreased 
but  little  by  the  change. 


RE-CONNECTING  INDUCTION  MOTORS  287 

(c)  The  running-light  current  will  be  slightly  lowered. 

(d)  The  starting  torque  will  probably  be  slightly  decreased. 

(e)  The  power-factor  might  be  expected  to  be  somewhat  higher. 

(/)  When  the  power-factor  is  higher  the  rating  of  the  motor  can  be 
increased  about  in  proportion  to  the  square  root  of  the  increase  in  speed 
with  the  same  heating. 

(flf)  The  efficiency  will  be  practically  the  same  as  before  the  change. 

Single -circuit  Delta  and  Double-circuit  Star  Connections. 

The  use  of  a  one-circuit  delta  and  a  double  or  two-circuit 
star  winding  for  a  three-phase  motor,  as  pointed  out  by  Henry 
Scheril  (Electrical  Record,  March,  1919)  depends  upon  operat- 
ing conditions  or  design  to  meet  certain  requirements.  One 
of  the  best  illustrations  is  offered  in  the  winding  of  a  phase- 
wound  rotor  of  the  induction  motor.  Suppose  that  the  rotor 
has  been  wound  with  a  two-circuit  star  winding  and  the  voltage 
between  rings  on  open  circuit  is  220  volts.  Let  us  assume  that 
the  full-load  current  in  the  rotor  is  200  amperes.  Since  the 
winding  has  a  double  circuit,  each  circuit  will  take  care  of 
100  amperes.  The  voltage  per  phase  will  be  127  volts. 

Manufacturers,  in  general,  standardize  the  brush  rigging 
used  in  connection  with  machines,  using  a  certain  number  of 
brushes  per  ring.  If  it  is  found,  for  instance,  that  200  amperes, 
as  in  this  example,  brings  the  current  density  in  the  brush  to 
too  high  a  value,  it  would  mean  that  the  number  of  brushes  per 
ring  must  be  increased.  Since  this  is  not  practical  nor  eco- 
nomical, a  change  in  the  connections  of  the  winding  may  bring 
about  the  desired  results,  that  is,  reduce  the  current  density 
in  the  brush  to  within  the  allowable  value. 

If  the  winding  were  then  reconnected  from  two-circuit  star 
to  single-circuit  delta,  then  the  voltage  between  terminals 
will  be  increased  from  220  volts  to  254  volts  and  the  current 
per  ring  would  be  reduced  from  200  amperes  to  173  amperes, 
or  a  reduction  of  13.5  per  cent.  This  reduction  may  just  be 
sufficient  to  decrease  the  brush  density  to  the  desired  value. 
Since  controllers  used  in  the  rotor  circuit  are  made  to  take  care 
of  voltages  of  reasonable  variations,  an  increase  in  voltage 
from  220  volts  to  254  volts  will  not  require  a  change  in  the 
controller  and  therefore  the  220  volts  controller  can  be  used 
on  the  254-volt  circuit. 


288         ARMATURE  WINDING  AND  MOTOR  REPAIR 

Cutting  out  Coils  of  an  Induction  Motor. — In  case  coils 
burn  out  in  an  induction  motor,  they  can  be  cut  out  and  the 
motor  operated  for  a  time  or  until  it  can  be  repaired.  It  is 
advisable  to  cut  the  entire  coil  and  tape  up  the  two  ends  so 
that  they  can  not  come  together.  There  is  a  limit  to  the 
number  of  coils  that  can  be  cut  out  as  this  is  equivalent  to 
raising  the  voltage  on  the  motor  which  will  cause  heating. 
Where  more  than  two  coils  must  be  cut  out  the  motor  should 
be  repaired  at  once  or  the  same  number  of  coils  in  each  phase 
cut  out,  evenly  distributed  around  the  stator. 

PROCEDURE  WHEN  CONNECTING  THE  COILS  OF  AN 
INDUCTION-MOTOR  WINDING 

At  the  end  of  this  chapter  typical  diagrams  for  connecting 
the  windings  of  polyphase  alternating-current  motors  are 
shown  as  prepared  by  A.  M.  Dudley.1  When  an  armature 
winder  or  repairman  knows  the  number  of  poles  and  phases 
for  the  winding,  and  has  been  supplied  with  the  necessary 
coils  of  the  proper  throw,  the  problem  of  inserting  the  coils 
and  then  connecting  them  up  can  be  easily  understood  for 
each  of  the  diagrams  referred  to  if  the  fundamental  procedure 
for  any  one  is  understood.  This  procedure  is  explained  by 
the  author  of  the  diagrams  as  follows: 

The  diagrams  are  not  dependent  on  the  total  number  of 
slots  in  the  machine  nor  upon  the  number  of  coils  per  group, 
nor  upon  the  throw  or  pitch  of  the  coils,  but  are  general  for 
all  machines  of  the  same  number  of  phases  and  poles.  Each 
one  of  the  small  arcs  in  each  diagram  represents  the  ends  of 
the  coils  in  a  single  pole-phase-group  in  the  winding.  This  is 
illustrated  in  Figs.  187  and  188  showing  a  stator  in  three  stages 
of  being  connected.  In  Fig.  187  (A)  a  machine  is  shown  in 
which  the  coils  have  simply  been  placed  in  the  slots  by  the 

1  The  diagrams  shown  have  been  selected  from  a  series  of  eighty-one 
devised  by  A.  M.  Dudley,  Engineer  industrial  division  of  the  Westing- 
house  Electric  &  Mfg.  Co.,  to  be  incorporated  in  an  excellent  book  by 
him  on  "Connecting  Induction  Motors."  (McGraw-Hill  Book  Co., 
New  York.)  Mr.  Dudley's  diagrams  begin  with  two-pole  windings  and 
give  practically  all  possible  combinations  for  two-  and  three-phase,  star 
and  delta  and  series-parallel  connections. 


RE-CONNECTING  INDUCTION  MOTORS 


289 


winder   and   no    connections   have   been   made.     The   wires 
which  are  the  beginnings  and  endings  of  the  coils  are  sticking 


I! 

,0      O 


3£ 


a  g^ 

^  rfl  CD 

a  ra  05 


M  'grt 

c3    8    § 

£o§ 


OO    O    O 

"-3 
s  +?  * 


put  at  random.     In  Fig.  187  (5)  the  coils  have  been  connected 
into  several  distinct  groups  and  the  remaining  wires,  which 

19 


290 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


protrude  radially  toward  and  away  from  the  center  of  the 
machine,  form  the  beginning  and  the  end  of  each  pole-phase- 
group. 

Connecting  Pole-phase-groups  of  a  Winding. — The  opera- 
tion which  has  been  performed  between  Fig.  187  (A)  and  Fig. 
187  (B)  can  be  described  in  this  way :  Suppose,  for  example,  that 
there  are  96  total  coils  in  the  winding  and  that  it  is  to  be  con- 
nected for  three  phases  and  four  poles.  There  will  then  be  3  X 


FIG.  188. — Stator  of  a  100-hp.,  3-phase,  8-pole,  220-volt,  60-cycle,  120- 
coil,  120-slot,  induction  motor  completely  wound  with  pole-phase-groups 
properly  cross  connected. 

4  =  12  pole-phase-groups,  and  96  -f-  12  =  8  coils  in  each  group. 
Starting  at  any  arbitrary  point,  the  winder  connects  the  first 
eight  coils  in  series  by  connecting  the  end  of  coil  No.  1  to  the 
beginning  of  coil  No.  2,  and  the  end  of  coil  No.  2  to  the  begin- 
ning of  coil  No.  3,  etc.,  until  eight  coils  are  in  series.  The 
beginning  of  coil  No.  1  is  then  bent  outward  and  left  long  and 
the  end  of  coil  No.  8  is  bent  inward  and  left  long.  Between 


RE-CONNECTING  INDUCTION  MOTORS 


291 


these  two  are  seven  short  " stubs"  or  coil-to-coil  connections, 
which  are  shown  taped  up  in  Fig.  187  (B) .  The  winder  then 
proceeds  to  connect  coils  No.  9  to  No.  16  in  series  in  the  same 
manner  to  form  pole-phase-group  No.  2,  and  so  on  around  the 
machine  until  he  has  completed  12  pole-phase-groups  and 
used  all  the  coils.  The  winding  then  looks  as  shown  in  Fig. 
187  (B). 

In  case  the  winding  has  certain  coils  provided  with  heavier 
insulation  on  the  end  turns  to  take  the  strain  of  the  full  voltage 


FIG.  189. 


FIG.  190. 


FIG.  191. 


FIG.  192. 


FIG.  193. 


FIG.  194. 


FIG.  189. — Thirty-two  coils  connected  into  8  pole-phase-groups  for  a 
2-phase  winding. 

FIG.  190. — Same  as  Fig.  189  with  pole-phase-groups  connected  according 
to  direction  of  arrows. 

FIG.  191. — Same  as  Fig.  190  with  B-phase  reversed.  » 

FIG.  192. — Forty-eight  coils  connected  into  12  pole-phase-groups  for  a 
3-phase  winding. 

FIG.  193. — Same  as  Fig.  192  with  pole-phase-groups  connected  according 
to  direction  of  arrows. 

FIG.  194. — Same  as  Fig.  193  except  leads  are  brought  out  from  different 
groups. 

of  the  machine  where  different  phases  are  adjacent,  the  opera- 
tion is  slightly  different.  In  this  case  the  number  of  coils  per 
pole-phase-group  must  be  determined  before  the  coils  are 
inserted  in  the  slots,  and  the  specially  insulated  phase  coils 
placed  on  both  ends  of  each  group.  In  this  case  the  location 


292         ARMATURE  WINDING  AND  MOTOR  REPAIR 

of  the  pole-phase-groups  is  definitely  determined  by  the  winder 
before  he  starts  connecting  the  coils  together. 

The  next  step  is  to  mark  the  pole-phase-groups  A-B-C-A- 
B-C,  etc.,  around  the  machine  and  then  to  connect  all  the 
groups  together  in  the  proper  manner  to  form  a  three-phase 
winding  as  indicated  by  the  diagram  of  connections.  The 
completed  winding  will  then  appear  as  shown  in  Fig.  188. 

General  Theory  on  which  Connection  Diagrams  are  Con- 
structed.— Simple  methods  by  which  any  winding  may  be 
checked  for  phase  polarity  are  shown  in  Figs.  189  to  194,  in- 
clusive. In  Fig.  189  a  winding  chosen  at  random  is  shown 
"stubbed"  into  pole-phase-groups  for  a  two-phase  connection, 
and  in  Fig.  192  stubbed  for  a  three-phase  connection.  To  de- 
termine the  proper  connections  for  the  pole-phase-groups  in  a 
two-phase  winding,  the  rule  is  to  mark  on  the  groups  arrows 
alternating  in  direction  in  pairs.  That  is,  on  two  successive 
groups  the  arrows  are  clockwise  and  on  the  two  immediately 
adjacent,  the  arrows  are  counter-clockwise.  Such  arrows, 
for  example,  are  shown  in  Fig.  189  just  above  the  windings. 
If  now  one  end  of  any  group  in  a  phase  is  chosen  as  a  "lead" 
and  all  the  groups  are  followed  through  and  connected  as  indi- 
cated by  the  arrows,  the  connection  will  be  correct.  Such  a 
connection  is  shown  in  Fig.  190.  However,  suppose  the  arrows 
had  alternated  in  pairs,  but  started  with  a  different  group,  as 
shown  just  below  the  windings  in  Fig.  190.  The  result  is  shown 
in  Fig.  191,  which  is  just  as  correct  as  Fig.  190,  except  that  the 
motor  would  run  with  the  opposite  direction  of  rotation. 
Since  the  rotation  can  be  changed  by  reversing  the  two  leads 
of  either  phase  outside  of  the  motor,  it  is  evident  that  the  rule 
using  the  arrows  alternating  in  pairs  is  correct  in  all  cases. 
It  should  also  be  noted  that  it  makes  no  difference  from  what 
group  the  lead  is  taken,  provided  all  the  groups  are  followed 
through  with  the  arrows. 

In  the  three-phase  machine  the  method  is  even  more  simple. 
The  rule  in  that  case  is  to  put  arrows  on  the  groups  alternating 
in  direction  from  group  to  group,  as  shown  in  Fig.  192.  Any 
group  may  then  be  chosen  as  a  "lead"  group  or  a  "star" 
group  so  long  as  the  arrows  are  followed  in  passing  from  the 
lead  to  the  star  in  each  phase.  Figure  193  shows  one  arrange- 


RE-CONNECTING  INDUCTION  MOTORS 


293 


ment  and  Fig.  194  another  equally  correct,  and  there  might  be 
an  indefinite  number  more,  simply  by  choosing  the  lead  from 
another  group  and  following  the  arrows  through  to  the  star 
in  each  phase.  Although  shown  for  a  developed  four-pole 
winding  only,  these  diagrams  may  be  considered  as  strictly 
general,  as  additional  groups  may  be  added  to  make  six,  eight, 
or  any  other  number  of  poles,  and  the  current  passed  through 
them  in  any  order,  so  long  as  the  phases  are  kept  in  the  correct 
rotation  and  the  current  in  the  right  direction  as  indicated 
by  the  arrows. 

In  case  of  a  delta  connection  instead  of  a  star,  check  the 
connections  through  as  for  a  star  and  then  connect  the  A  star 


FIG.  195. — Method  of  checking 
a  delta  connection  from  a  star 
connection. 


FIG.  196. — Winding  diagram  for 
2-pole,  2-phase  motor  with  series 
connections  of  coils. 


to  the  B  lead,  the  B  star  to  the  C  lead,  and  the  C  star  to  the  A 
lead,  as  shown  in  Fig.  195;  or  connect  the  A  lead  to  the  B 
neutral,  the  B  lead  to  the  C  neutral,  and  the  C  lead  to  the  A 
neutral.  The  three  motor  leads  will  be  taken  from  the  corners 
of  the  delta  so  formed. 

Determining  Number  of  Poles  from  Slot  Throw  of  Coils. — 
As  an  example  assume  a  96-slot  stator  whose  coils  span  12 
slots.  Then  number  of  slots  -t-  span  of  coil  =  number  of 
poles,  or  96  -f-  12  =  8  poles.  Suppose  however,  the  coils 
span  10  slots.  The  quotient  is  then  9.6  which  is  an  impossible 
number  of  poles.  This  indicates  a  chorded  winding  and  the 


294         ARMATURE  WINDING  AND  MOTOR  REPAIR 

correct  number  of  poles  is  probably  the  next  lower  even 
number  which  will  again  be  8.  This  is  not  an  invariably 
correct  rule.  A  further  check  is  as  follows:  Divide  the 
number  of  slots  by  the  number  of  phases.  If  this  number 
is  divisible  by  the  number  of  poles  obtained  as  above,  it  may 
be  safely  assumed  that  the  correct  number  of  poles  has  been 
determined. 

Typical  Circle  Diagrams  for  Connecting  Induction  Motors. 
On  the  following  pages,  295  to  300  typical  winding  diagrams 
are  shown  by  which  induction  motors  may  be  connected  for 
a  variety  of  operating  conditions. 


RE-CONNECTING  INDUCTION  MOTORS 


295 


FIG.  197. — Winding  diagram  for 
2-pole,  3-phase  motor  with  series-star 
connection  of  coils. 


FIG.  198. — Winding  diagram  for 
4-pole,  2-phase  motor  with  series  con- 
nections of  coils. 


B 


FIG.  199. — Winding  diagram  for 
4-pole,  3-phase  motor  with  series- 
star  connections  of  coils. 


FIG.  200. — Winding  diagram  for 
4-pole,  3-phase  motor  with  series- 
delta  connections  of  coils. 


296 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


CAB 


A    C 


FIG.    201. — Winding    diagram    for  FIG.    202. — Winding    diagram    for 

4-pole,  3-phase  motor  with  2-parallel       4-pole,  3-phase  motor  with  2-parallel 
star  connections  of  coils.  delta  connections  of  coils. 


FIG.   203. — Winding    diagram    for  FIG.    204. — Winding    diagram    for 

4-pole,  2-phase  motor  with  4-parallel       6-pole,  2-phase  motor  with  series  con- 
connections  of  coils.  nections  of  coils. 


RE-CONNECTING  INDUCTION  MOTORS 


297 


FIG.  205. — Winding  diagram  for 
6-pole,  3-phase  motor  with  series-star 
connections  of  coils. 


B  C 


FIG.  206.  —  Winding  diagram  for 
6-pole,  3-phase  motor  with  series- 
delta  connections  of  coils. 


B  C 


FIG.  207. — Winding  diagram  for 
6-pole,  3-phase  motor  with  2-parallel 
star  connections  of  coils. 


ABC 

FIG.  208. — Winding  diagram  for 
6-pole,  3-phase  motor  with  2-parallel 
delta  connections  of  coils. 


298         ARMATURE  WINDING  AND  MOTOR  REPAIR 


ABC 

FIG.    209. — Winding    diagram    for  FIG.    210. — Winding    diagram    for 

6-pole,  3-phase  motor  with  3-parallel  6-pole,  3-phase  motor  with  6-parallel 

delta  connections  of  coils.  delta  connections  of  coils. 


FIG.    211. — Winding   diagram   for  FIG.    212. — Winding    diagram    for 

8-pole,  2-phase  motor  with  series  con-       8-pole,  2-phase  motor  with  8-parallel 
nections  of  coils.  connections  of  coils. 


RE-CONNECTING  INDUCTION  MOTORS 


299 


ABC  ABC 

FIG.  213. — Winding  diagram  for  FIG.  214. — Winding  diagram  for 
8-pole,  3-phase  motor  with  series-star  8-pole,  3-phase  motor  with  series- 
connections  of  coils.  delta  connections  of  coils. 


" 


i  ... 

i 

13X  v 
.9 
17                         11 
15                               j 

£-1-,-^ 

•1lH-t-f-^~l 

1 

9 

4  •'• 

17 

H?  . 

i 
i 

:;    .  ^ 

FIG.    215. — Winding    diagram    for  FIG.    216. — Winding    diagram   for 

10-pole,   2-phase    motor   with   series       10-pole,    3-phase    motor  with  series- 
connections  of  coils.  star  connections  of  coils. 


300 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


ABC 

FIG.    217. — Winding    diagram    for  FIG.    218. — Winding   diagram   for 

10-pole,   3-phase  motor  with  series-       10-pole,  3-phase  motor  with  5-parallel 
delta  connections  of  coils.  star  connections  of  coils. 


B  C 


FIG.  219. — Winding  diagram  for 
10-pole,  3-phase  motor  with  5-parallel 
delta  connections  of  coils. 


ABC 

FIG.  220.— Winding  diagram  for 
10-pole,  3-phase  motor  with  10-par- 
allel  star  connections  of  coils. 


CHAPTER  XII 
COMMUTATOR  REPAIRS 

Commutator  troubles  are  more  easily  located  than  faults  in 
an  armature,  but  a  repair  job  on  an  old  style  motor,  many  of 
which  are  still  in  use,  is  sometimes  a  trying  and  tedious 
operation. 

Causes  of  Commutator  Troubles.  —  Sparking  at  the  brushes 
is  generally  the  first  symptom  of  commutator  trouble.  One 
of  the  most  frequent  causes  of  sparking  is  a  rough  or  pitted 
commutator.  This  may  be  due  to  many  irregularities,  such 
as  overload;  brushes  out  of  line;  not  set  at  neutral  points  in 
regard  to  load;  poor  contacts;  current  density  per  square  inch 
of  brush  contact  too  great;  open  circuit;  weak  magnetic 
fields;  commutator  out  of  round;  high  or  low  bars  or  high  mica. 
The  most  common  cause  of  sparking  is  high  mica  which  causes 
the  brushes  to  chatter  and  to  make  poor 
contact.  This  condition  results  in  a 
rapid  blackening  and  burning  of  the 
bars,  sometimes  to  the  extent  that  the 
copper  is  eaten  away  leaving  the  mica 
segments  standing  out  above  the  surface 
of  the  commutator.  Some  motors  seem 
to  be  particularly  subject  to  this 
trouble,  due  to  the  fact  that  the  mica 
is  too  hard  a  grade,  or  the  copper  too 
soft.  When  there  is  no  time  to  turn 
down  the  commutator  the  high  mica  and  how  they  are  held  by 
can  be  removed  by  grinding  down  the  clamPins  rings- 
with  a  piece  of  sandstone,  and  using  fine  sandpaper  for 
smoothing. 

Troubles  Resulting  from  High  Mica.  —  High  mica,  while 
it  may  seem  a  small  matter  at  first,  is  often  the  cause  of  more 
serious  complications.  The  commutator  may  become  so 

301 


SEGMENT 


FIG.  221.  —  Section  of  a 


302         ARMATURE  WINDING  AND  MOTOR  REPAIR 

hot  from  the  poor  brush  contact  afforded,  that  the  solder 
will  be  melted  and  thrown  out,  resulting  in  short-circuits 
between  bars  and  open-circuits  due  to  the  armature  leads 
becoming  disconnected.  About  the  only  permanent  relief  for 
sparking  at  brushes  due  to  high  mica  is  undercutting  the  mica. 
This  remedy  is  recommended  when  it  is  reasonably  certain  that 
the  high  mica  is  caused  by  the  natural  condition  of  the  copper 
or  mica.  If  it  is  not,  then  the  real  cause  must  be  found, 
otherwise,  undercutting  the  mica  would  probably  improve 
the  running  condition  somewhat,  but  would  fail  to  remove  the 
cause. 

The  mica  should  not  be  cut  too  deeply,  a  depth  of  ^2  to  KG 
inch  below  the  surface  of  the  copper  being  sufficient.  Care 
must  be  exercised  to  remove  the  mica  the  full  width  of  the 
segment,  for  any  thin  slivers  left  flush  with  the  surface  will 
often  defeat  the  purpose  of  the  undercutting.  (For  details 
for  undercutting  mica,  see  page  320.)  This  method  has 
corrected  some  stubborn  cases  of  sparking  and  if  the  job  is 
properly  done,  all  that  will  be  necessary  to  preserve  sparkless 
commutation  is  to  keep  the  slots  clean  and  well  below  the 
surface  of  the  copper. 

Remedy  for  High  or  Low  Bars. — A  new  or  repaired  motor 
may  have  a  commutator  that  is  not  " settled."  That  is,  the 
clamping  ring  has  not  been  drawn  up  as  tightly  as  it  should  be. 
When  the  mica  end  rings  are  "cut  they  are  only  slightly  flexible 
due  to  the  shellac  in  them  and  cannot  be  made  to  fit  perfectly 
when  cold.  When  the  commutator  is  hot,  the  shellac  in  the 
mica  will  soften  and  allow  it  to  move  under  the  strain  of  the 
centrifugal  force  of  the  bars  when  the  machine  is  running. 
This  movement  of  the  mica  allows  the  bars  to  move  and  is 
frequently  the  cause  of  high  or  low  bars.  To  remedy  this 
trouble,  the  machine  should  be  run  until  its  normal  operating 
temperature  is  attained  and  then  shut  down.  The  clamp- 
ing ring  can  then  be  tightened.  This  process  may  be  neces- 
sary several  times,  or  until  the  commutator  is  perfectly  solid. 
Care  must  be  taken  not  to  tighten  the  bolts  too  much  while 
the  commutator  is  warm. 

In  the  case  of  a  high  bar,  it  should  be  tapped  down  until  it 
rests  firmly  against  the  mica  end  rings.  It  can  then  be  filed 


COMMUTATOR  REPAIRS 


303 


even  with  the  rest  of  the  bars.  A  low  bar  can  be  raised  by 
prying  up,  and  inserting  a  narrow  strip  of  mica  beneath  it, 
but  in  the  majority  of  cases  this  makes  a  poor  job.  Usually 
the  only  alternative  is  to  turn  the  commutator  down  to  the 
level  of  the  low  bar. 

Burn-out  Between  Bars. — Probably  the  most  frequent  com- 
mutator trouble  is  a  burn-out  between  bars.  It  occurs  often 
on  the  corner  of  the  bars,  and  is  not  infrequently  caused  by 


FIG.  222. — Mica  segments  taken  from  damaged  motors  showing  the  effects 
of  short  circuits  in  the  commutator.  Two  tools  are  also  shown  made  from 
hack  saw  blades  for  use  in  plugging  a  commutator. 

oil  working  along  the  shaft  from  the  bearing  and  up  onto  the 
commutator.  This  oil  collects  dust  and  dirt  and  finally 
causes  current  to  leak  from  one  bar  to  the  other.  The  mica 
then  becomes  carbonized,  and  a  short-circuit  results.  This 
is  one  of  the  causes  of  burned-out  armature  coils.  Sometimes 
the  short-circuit  will  burn  itself  clear,  and  no  harm  will  be 
caused  except  to  burn  a  hole  in  the  mica.  However,  it  may 


304         ARMATURE  WINDING  AND  MOTOR  REPAIR 

continue  to  arc  across  and  burn  a  good  sized  hole  in  the  bars 
also. 

Plugging  a  Commutator. — When  mica  segments  are  burned 
but  not  too  deep,  the  holes  can  be  cleaned  with  a  thin  knife 
blade  and  plugged  with  some  kind  of  filling.  If  a  good  filling 
compound  is  used,  and  the  commutator  kept  free  from  oil,  it 
will  hold  for  a  year  or  possibly  longer.  It  is  always  advisable 
to  save  the  wearing  surface  in  this  manner  whenever  it  can  be 
done,  for  every  time  a  commutator  is  turned  down  in  a  lathe 
on  an  average  of  three  years  of  its  useful  life  is  lost. 

A  good  filling  compound  for  commutators  can  be  made  as 
follows:  Two  parts  plaster-of-paris;  one  part  powdered  mica; 
and  enough  glue  to  make  a  thick  paste.  This,  when  applied, 
will  dry  quickly,  and  assume  about  the  same  degree  of  hard- 
ness as  the  mica  segments. 

When  a  segment  becomes  burned  deep  down  into  the  com- 
mutator, a  new  one  must  be  inserted.  Before  attempting  to 
do  this,  the  armature  should  be  thoroughly  blown  out  with  com- 
pressed air  in  order  to  remove  all  dust  that  may  have  accu- 
mulated. This  is  essential,  for  it  is  an  easy  matter  for  small 
particles  of  foreign  matter  to  work  in  under  the  back  end  be- 
tween the  bars  and  the  sleeve  when  the  commutator  is  loose. 
Determine  just  which  mica  segments  must  be  taken  out,  and 
number  the  bars  at  each  burn-out,  since  the  bars  may  have  to 
be  removed  also  in  order  to  get  the  mica  segments  out.  If 
the  segments  were  not  shellaced  when  the  commutator  was 
built,  the  chances  are  they  can  be  lifted  out  without  disturb- 
ing the  bars.  Otherwise  the  bars  will  have  to  be  taken  out 
with  the  segments. 

Removing  Bars  and  Mica  Segments  for  Repairs. — Remove 
the  bolts  that  hold  the  clamping  ring  in  place.  Mark  the 
ring  so  that  it  may  be  put  back  just  as  it  was  taken  off.  Tap 
the  end  of  the  ring  lightly  with  a  hammer.  If  the  mica  ring 
does  not  loosen  from  the  commutator,  it  will  have  to  be  heated, 
as  the  ring  is  probably  stuck  fast  with  shellac.  Heat  the 
commutator  with  a  torch  to  an  even  temperature  all  around. 
This  will  expand  the  copper  and  cause  it  to  bulge  out  from  the 
end  ring.  Tap  the  ring  again  lightly,  and  it  will  be  found  to 
work  loose.  Then  it  can  be  pulled  out. 


COMMUTATOR  REPAIRS 


305 


Pry  the  bars  apart  slightly  at  one  of  the  burned  places  to 
see  if  the  mica  segments  are  stuck  to  the  bars.  If  they  are 
not,  it  is  a  simple  matter  to  remove  them  from  the  commu- 
tator. If  they  are  held  fast,  the  leads  from  one  bar  adjoining 


Keep  all  Dirt  and 
Moisture  out 


Leave  Mica  extending 
One-sixteenth  Inch 


Seal  thoroughly  against 
Dirt  and  Moisture 


FIG.  223. — Section  of  an  assembled  commutator  of  a  railway  motor. 

the  burned  segment  should  be  unsoldered  and  the  bar  lifted 
out.  Proceed  in  the  same  way  with  the  remaining  bad  places. 
New  segments  can  be  marked  off  by  using  one  of  the  old  bars 
as  a  guide.  The  bars  should  be  scraped  and  filed  clean^ 
and  all  rough  corners  rounded  off. 

Repairing    a    Burned    Commutator    Bar. — Frequently    it 
happens  that  there  is  a  good-sized  hole  burned  in  the  commu- 


Fio.  224. — Repair  of  a  burned  place  on  a  commutator  bar. 

tator  bar.  This  should  be  repaired  before  it  is  used  again. 
As  there  is  usually  no  stock  of  exactly  the  proper  size  on  hand 
in  a  small  repair  shop  out  of  which  to  make  a  new  bar,  a  good 
repair  is  the  next  best  thing.  In  Fig.  224  (A)  shows  the  end  of 
20 


306         ARMATURE  WINDING  AND  MOTOR  REPAIR 

a  bar  with  a  burned  place  and  (B)  the  method  of  repair.  In 
this  case  the  bar  was  cut  down  enough  to  remove  the  burn, 
and  a  piece  of  copper  strip  carefully  squared  and  soldered  in 
the  cut.  It  was  then  riveted  with  a  small  copper  rivet,  the 
location  of  which  is  shown  at  (X).  The  rivet  was  used  to 
prevent  the  patch  from  flying  out  should  the  commutator  for 
any  reason  become  hot  enough  to  melt  the  solder.  Such  a 
patch  should  be  filed  down  to  the  dimensions  of  the  bar.  The 
cut  for  a  patch  of  this  kind  can  be  quickly  made  if  a  milling 
machine  or  shaper  is  handy,  otherwise  a  good  sharp  fils  will  serve 
the  purpose. 

Replacing  a  Repaired  Commutator  Bar. — Before  replacing 
a  commutator  bar  that  has  been  repaired,  an  inspection  should 
be  made  of  the  back  mica  ring,  to  be  sure  that  no  dust  or  solder 
has  lodged  there.  The  mica  segments  should  be  replaced 
first,  and  then  the  bar  pushed  in.  Shape  the  commutator 
as  nearly  as  possible  into  a  circular  form  and  replace  the  end 
ring.  Tighten  the  clamping  nuts  as  much  as  possible  while 
the  commutator  is  cold.  It  is  a  good  plan  to  paint  the  end 
of  the  commutator  with  shellac,  in  order  to  fill  up  any  cracks 
that  may  exist  between  the  bars  and  the  ring. 

Tightening  up  a  Repaired  Commutator. — When  a  commu- 
tator has  been  taken  down  in  a  repair  shop  and  assembled 
again,  all  lock  nuts  and  screws  should  be  first  set  up  hard 
and  the  commutator  baked  in  an  oven.  Then  the  lock  nuts 
and  screws  can  be  tightened  up  again  since  the  heating  causes 
the  copper  to  expand  and  put  pressure  on  the  mica  which, 
combined  with  the  heat,  drives  out  all  traces  of  shellac  in  the 
mica.  Then  as  the  commutator  cools  and  the  copper  becomes 
normal  the  bolts  can  be  taken  up.  This  proces's  should  be 
repeated  and  finally  the  commutator  cooled  quickly  by  a  fan, 
and  the  nuts  tried  again.  If  they  seem  tight  the  commutator 
is  ready  for  assembly  on  the  armature  shaft.  It  requires  some 
experience  to  determine  just  how  tight  the  bolts  can  be  drawn 
on  a  commutator  without  injuring  the  mica  end  rings.  The 
one  precaution  is  not  to  draw  up  the  bolts  finally  until  the 
commutator  has  cooled.  The  commutator  surface  can  now 
be  turned  down  in  a  lathe.  After  doing  this  a  short-circuit 
test  should  be  made  with  a  test  lamp  from  bar  to  bar.  Failure. 


COMMUTATOR  REPAIRS 


307 


308          ARMATURE  WINDING  AND  MOTOR  REPAIR 

of  the  lamp  to  light  indicates  that  the  commutator  is  free  from 
short-circuits.  See  also  Chapter  V,  pages  123,  126  and  135. 
A  test  for  grounds  should  also  be  made  at  this  time  (see  pages 
131  and  175). 

Baking  Commutator  with  Electric  Heat. — In  those  cases 
where  an  oven  is  not  available  for  baking  a  rebuilt  commutator 
and  it  is  too  large  to  heat  with  a  torch,  the  electrical  method 
shown  in  Fig.  226  has  been  successfully  used  (Albert  Krause, 
ElectricalWorld,  July26,  1919,  page  190)  A  satisfactory  heat- 
ing element  can  be  made  up  by  using  a  layer  of  ^{Q-UI.  asbestos 
paper  around  the  commutator  and  winding  over  this  about 


FIG.  226. — Uniform  temperature  produced  by  heating  element  wound  around 

commutator. 

90  ft.  of  No.  22  Nichrome  resistance  wire.  By  applying 
250  volts  to  the  terminals  of  this  resistance  wire  a  uniform 
heat  in  the  commutator  can  be  produced  to  permit  evening  up 
the  insulating  segments  and  clamping  the  end  rings.  Ordi- 
narily it  would  be  advisable  to  have  a  variable  resistance  in 
series  with  the  heating  element  so  its  temperature  can  be  kept 
at  a  desirable  value.  By  covering  the  armature  after  the 
commutator  is  heated  in  this  manner  varnish  may  be  applied 
thereto  and  baked  in  by  the  heat  conducted  to  the  armature 
through  the  leads  from  the  commutator. 
Removing  and  Repairing  Grounds  in  a  Commutator. — The 


COMMUTATOR  REPAIRS  309 

ground  frequently  occurs  between  the  sleeve  ring  and  the  end 
of  the  bars.  A  small  hole  is  generally  burned  through  the 
mica  ring  or  taper  cone.  Some  times  the  mica  ring  on  the  rear 
end  is  punctured.  In  that  event  a  number  of  bars  in  the 
neighborhood  of  the  ground  will  have  to  be  taken  out.  The 
burned  mica  should  be  cut  out 
and  a  patch  put  on.  When  the 
trouble  occurs  on  the  front  end 
of  the  commutator,  remove  the 
ring  and  cut  out  the  bad  mica. 
The  patch  can  be  made  as  shown 

TV        rkr*>-r        mi  •  •  ±.      FIG.  227. — Patch  on  a  mica  end 

in  Fig.  227.     This  new  mica  must          ring  of  a  commutator. 
be  a  trifle  thicker  than  the  original 

mica  removed,  for  it  will  squeeze  together  somewhat  when 
the  ring  is  drawn  up  tight  and  the  commutator  heated. 

After  the  repair,  test  the  commutator  for  grounds  with  a 
proper  voltage  (see  page  175) .  Place  one  terminal  on  the  shaft, 
and  move  the  other  completely  around  the  surface  of  the  com- 
mutator. Freedom  from  grounds  will  ,be  indicated  by  no 
sparking  at  the  terminal  moved  over  the  surface.  A  lower 
test  voltage  should  be  used  on  low- voltage  commutators  and 
a  higher  voltage  for  those  of  high  voltage  machines. 

On  some  of  the  old-style  motors,  the  sleeve  nut  is  on  the 
back  of  the  commutator.  When  this  happens,  the  armature 
leads  will  have  to  be  disconnected  and  bent  back  out  of  the 
way  in  order  to  work  on  the  commutator.  The  best  proce- 
dure in  such  a  case,  if  there  is  time,  is  to  remove  the  com- 
mutator and  reverse  the  sleeve,  as  both  ends  are  usually  bored 
to  the  same  diameter.  By  reversing  the  sleeve  in  this  manner 
in  order  to  get  the  nut  where  it  can  be  easily  reached,  much 
time  and  work  will  be  saved  when  future  repairs  must  be  made 

Turning  Down  a  Commutator  without  Removing  Armature 
from  Machine. — The  method  sometimes  used  in  turning  down 
a  commutator  on  a  repair  job  where  the  armature  is  too  large 
to  remove,  is  to  leave  one  or  two  pairs  of  brush  arms  on  and 
run  the  machine  from  these  at  as  low  a  speed  as  the  field  regu- 
lation will  permit  or  possibly  with  a  water  rheostat  in  the  arma- 
ture circuit.  This  method  will  do  where  no  other  means  of 
turning  the  commutator  is  available.  There  is  always  bad 


310         ARMATURE  WINDING  AND  MOTOR  REPAIR 

sparking  and  burning  at  the  point  of  the  cutting  tool  due 
to  its  short-circuiting  the  bars  when  it  crosses  the  mica.  The 
tool  has  to  be  sharpened  frequently  and  a  job  is  seldom  good 
even  where  the  greatest  care  is  exercised. 

For  these  reasons  it  is  always  preferable  to  belt  the  machine 
to  a  separate  motor  and  turn  down  the  commutator  with 
the  fields  unexcited.  In  connecting  up  a  motor  for  driving 
the  armature  the  speed  should  be  made  as  low  as  possible, 
preferably  not  over  75  revolutions  per  minute. 

Temporary  Cover  for  Use  When  Turning  Down  a  Commu- 
tator.— In  cases  where  it  is  necessary  to  turn  down  the  commu- 
tator of  a  direct- current  machine  without  removing  it  from 


ir First  Cord  on  Cover 


2r Cover  Stretched  up  Over  Leads. 


a-  Lost  Tie  on  Cover  4i-  Last  End  Pulled  under 

Cord  Winding. 

FIG.  228. — Steps  in  applying  a  cotton  cover  over  the  end  of  an  armature 
before  turning  down  the  commutator  to  prevent  copper  chips  falling  behind 
the  bars  to  cause  short  circuits. 


the  frame,  H.  S.  Rich  has  made  use  of  a  muslin  cover  (Elec- 
trical Record,  December,  1918)  tied  over  the  open  leads  so  that 
copper  chips  cannot  find  a  way  down  behind  the  bars  to  cause 
a  short-circuit.  This  cover  is  made  and  applied  as  follows: 
Cut  a  strip  of  muslin  wide  enough  to  reach  across  the  commu- 
tator and  well  past  the  leads,  and  long  enough  to  go  around 
one  and  one-half  times.  Tie  this  very  securely  with  fine 
strong  cord,  with  the  muslin  laid  over  the  bars  as  shown  in 
Fig.  228. 

Draw  the  covering  and  stretch  a  little  at  a  time  all  around 
and  up  over  the  open  leads  far  enough  back  to  allow  of  at  least 
two  separate  cords  to  be  tightly  wound  around  and  very  se- 


COMMUTATOR  REPAIRS  311 

jfe 
curely  tied  on  the  core  body.     Turn  the  armature  slowly  to 

see  that  the  covering  does  not  interfere  with  any  brush  holder. 
If  so,  the  holder  should  be  shifted.  The  surplus  edging,  all 
loose  strings  and  threads  should  now  be  trimmed  off  all  around. 

By  turning  back  the  muslin  over  the  leads  the  first  cord 
tied  around  the  bars  is  covered  neatly.  The  cords  wound 
around  the  core  body  can  be  secured  without  knots  by  wind- 
ing over  the  first  end  a  few  turns,  and  then  by  winding  over 
a  short  extra  loop,  the  last  end  can  be  jerked  under  all  the 
turns  and  cut  short  so  that  a  knot  is  not  needed.  For  a 
permanent  armature  covering,  shellac  should  be  applied  all 
over  it  which  seals  the  cords  and  stiffens  the  muslin. 

Refilling  a  Commutator. — When  a  commutator  is  to  be 
refilled,  disconnect  the  armature  leads  and  remove  the  com- 
mutator. A  simple  device  for  accomplishing  this  is  shown 
in  Fig.  229.  Two  long  bolt  rods  are  screwed  into  holes  tapped 
into  the  sleeve  ring,  and  a  bar  of  heavy  iron  placed  across  the 
end  of  the  shaft,  with  bolt  rods  coming  through,  as  shown. 
By  tightening  the  nuts  evenly,  the  commutator  can  be  pulled 
off.  Next  count  the  number  of  bars  carefully,  and  enter 
this  in  a  note  book  for  future  reference.  Remove  the  sleeve 
and  if  possible  save  the  mica  rings.  If  these  are  in  good 
condition  they  can  be  used  again.  Carefully  caliper  the 
diameter  of  these  rings  and  enter  this  in  the  note  book  also, 
as  the  new  commutator  will  have  to  be  bored  to  fit  these  rings. 

With  a  micrometer  caliper, 
measure  the  thick  and  thin 
edges  of  one  of  the  bars  in 
thousands  of  an  inch;  also  the 

thickness  Of  the  mica  Segment.  Fl"  229.-Device  for  removing 
It  is  advisable  to  Order  the  a  commutator  from  an  armature 

bars  and  mica  segments  from 

the  motor  manufacturer  sawed  to  the  proper  size,  as  in  all 
probability  this  can  be  done  cheaper  than  in  an  ordinary  repair 
shop  not  equipped  for  this  work.  When  ordering  new  bars 
a  detailed  drawing  should  be  sent,  giving  all  necessary 
dimensions  of  the  old  bar  for  boring  and  turning  purposes. 
Hard  drawn  copper  is  usually  used  as  it  wears  at  about  the 
same  rate  as  the  mica  segments  (see  page  319). 


312         ARMATURE  WINDING  AND  MOTOR  REPAIR 

Use  of  a  Commutator  Clamp. — When  assembling  the  com- 
mutator, a  clamp  will  be  necessary  to  hold  the  bars  to- 
gether while  boring.  Several  makeshift  methods  are  available, 
but  it  will  pay  any  repair  shop  to  have  suitable  cast-iron 
clamps,  such  as  shown  in  Fig.  230.  The  clamp  should  be 
smaller  than  the  diameter  of  the  commutator,  so  that  when 
it  is  drawn  tight,  there  will  be  a  space  of  about  %  inch  between 
the  sections.  When  using  this  clamp  as  shown  in  Fig.  230 
at  the  right,  wooden  blocks  (C)  can  be  employed  to  hold  the 
clamp  about  midway  of  the  commutator.  D  is  an  iron  face 
plate.  The  clamp  (B)  should  first  be  placed  on  the  plate  as 
shown,  and  the  bars  and  mica  segments  stacked  in  a  circular 
form  within  it.  Care  must  be  exercised  to  make  sure  that  a 


FIG.  230. — At  the  left,  a  clamp  for  holding  commutator  bars  together  when 
being  assembled.     At  the  right,  the  use  of  this  clamp  is  shown. 

mica  segment  is  placed  between  each  copper  bar.  Count 
the  bars  carefully,  so  that  their  number  corresponds  with  the 
number  of  bars  in  the  original  commutator. 

Take  several  pieces  of  copper  wire  (about  No.  9  B.  &  S. 
gauge)  and  remove  the  insulation.  Place  these  around  the 
commutator  near  the  top  and  lower  ends  to  act  as  band  wires, 
and  twist  them  tight.  The  clamp  may  then  be  removed, 
and  the  commutator  straightened.  Bring  out  the  mica 
segments  even  with  the  surface  of  the  bars  by  holding  the 
fingers  against  the  inside  edge  of  the  segments  and  tapping 
the  bars  on  the  outside  with  a  small  mallet.  Place  a  square 
or  steel  scale  on  the  face  plate  and  tap  the  bars  on  the  outside 
with  a  small  mallet.  Place  the  square  or  steel  scale  on  the 
face  plate  and  see  that  the  bars  line  up  perpendicularly  with 
one  edge  of  the  square.  If  they  do  not,  a  gentle  pressure  one 


COMMUTATOR  REPAIRS  313 

way  or  the  other  on  the  top  end  of  the  commutator  with  the 
palm  of  the  hand  will  bring  them  in  line.  See  that  each  bar 
and  segment  is  down  flat  against  the  surface  of  the  plate, 
since  that  end  will  be  fastened  to  the  face  plate  on  the  lathe 
when  facing  off  the  ends  of  the  bars.  Tap  each  bar  and  seg- 
ment down  solid  with  a  square  ended  punch,  a  little  narrower 
than  the  thickness  of  the  bar.  When  this  has  been  done,  the 
band  wires  can  be  drawn  a  little  tighter,  and  the  surface  of 
the  commutator,  where  the  clamp  will  fit,  should  be  filed 
to  remove  any  protruding  mica,  and  present  a  smooth  surface 
for  the  clamp. 

Replace  each  section  of  the  clamp  about  the  commutator 
again  using  the  wooden  blocks  mentioned  before.  Draw 
the  clamp  tight,  being  sure  to  leave  the  same  amount  of 
space  between  each  clamp  section.  A  small  gas  burner, 
or  some  other  source  of  heat  should  be  handy,  and  the  com- 
mutator placed  over  it  and  heated.  When  it  is  good  and 
hot  to  the  hand,  tighten  the  clamp,  allow  it  to  cool,  and  again 
tighten. 

Boring  out  the  End  of  a  Commutator. — The  next  thing  to  do 
is  to  bolt  the  commutator  to  the  face  plate  of  the  lathe  and 
center  it.  The  same  wooden  blocks  can  be  used  again  for 
supporting  the  clamp.  Face  off  the  end  of  the  bars,  and  then 
groove  out  the  end  for  the  taper  rings  to  the  same  diameter 
as  the  mica  rings  on  the  sleeve.  Take  one  of  the  old  bars, 
and  with  a  bevel  protractor  determine  exactly  the  taper  used 
on  the  old  commutator.  The  usual  taper  employed  is  shown 
in  Fig.  231.  A  small  groove  (shown  at  A)  should  be  cut  below 
the  intersection  of  the  two  tapers  to  allow  room  for  the  edge 
of  the  mica  ring.  The  other  end  should  be  treated  in  the  same 
manner. 

When  the  boring  has  been  completed,  a  close  inspection 
must  be  made  of  all  turned  surfaces,  to  make  certain  that  no 
copper  has  been  dragged  over  the  mica  to  form  a  short- 
circuit.  The  corner  of  the  groove  A  (Fig.  231)  should  be 
carefully  gone  over,  as  it  is  here  that  drag-overs  most  fre- 
quently occur.  Scrape  and  wipe  the  mica  rings  on  the  sleeve 
clean;  also  wipe  out  the  inside  of  the  commutator  with  a  soft 
rag.  Place  the  sleeve  in  the  commutator  again,  and  draw 


314 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


up  on  the  end  nut.  On  a  refilling  job,  when  the  old  mica 
rings  are  used  again,  cracks  may  be  found  between  the  band 
ring  and  the  commutator  copper  due  to  the  irregularity  of 
the  mica  ring  caused  by  the  shape  of  the  old  commutator. 
These  cracks  can  be  filled  by  pushing  in  thin  sheets  of  mica. 
Shellac  should  be  used  liberally  on  the  ends. 

Bake  the  commutator  in  an  oven  for  about  one  hour  until 
it  becomes  thoroughly  hot,  and  tighten  the  nuts.  Reduce 
the  heat  somewhat,  and  bake  at  a  low  heat  until  the  shellac 


II 


Ill 

FIG.  231. — I  shows  taper  of  commutator  bars.  II  is  a  template  for  laying 
out  mica  end  rings.  Ill  shows  a  scheme  for  laying  out  the  taper  E,  of  the 
template  shown  in  II. 

becomes  hard.  Tighten  the  nuts  again  and  allow  the  com- 
mutator to  cool.  When  cold  give  the  nuts  a  final  setting  up. 
The  clamp  can  now  be  removed. 

In  order  to  finish  the  commutator,  use  a  mandrel  to  fit 
the  bore  of  the  sleeve,  and  take  the  final  finishing  cuts.  After 
finishing,  a  bar  to  bar  test  with  the  test  lamp  outfit  should 
be  made  to  be  sure  there  are  no  short-circuits  between  bars. 
A  test  for  grounds  should  also  be  made  by  holding  one  wire 
of  the  test  lamp  on  the  iron  sleeve  and  passing  the  other  from 
bar  to  bar  around  the  commutator.  A  high-voltage  test  for 
grounds  should  be  made  also  later. 


COMMUTATOR  REPAIRS 


315 


The  slots  for  taking  the  armature  leads  must  next  be  cut. 
It  is  well  to  cut  them  a  little  wider  than  the  diameter  of  the 
lead  wires.  These  slots  should  be  made  in  a  milling  machine 
if  one  is  at  hand,  otherwise  a  hack  saw  can  be  used.  Some- 
times two  blades  fastened  together  will  give  the  required 
width  of  slot.  The  commutator  can  now  be  placed  on  the 
armature  shaft. 

Mica  Used  in  Commutators. — The  selection  of  mica  for  the 
insulation  of  commutator  bars  and  for  V-rings  is  usually  based 
upon  the  type  of  motor  being  repaired,  but  built-up  mica  has 
been  found  to  be  the  most  satisfactory.  India  mica  is  gener- 
ally too  hard  and  domestic  mica  too  brittle  for  use  between 


FIG.  232. — Mica  segment  (B)  cut  from  sheet  using  bar  (A)  as  pattern. 
Such  a  segment  is  cut  large  at  top  and  at  ends  so  as  to  turn  down  evenly 
with  copper  bars  when  commutator  is  finally  surfaced. 

bars.  Domestic  mica  also  has  cracks  and  fissures  which  fill 
up  with  dirt  and  cause  short  circuits.  Amber  mica  gives 
good  service  but  it  comes  in  such  odd  sizes  that  it  is  too 
wasteful  for  the  average  repair  job.  Hard-baked  built-up 
mica  is  found  best  to  be  used  between  bars  and  the  unbaked 
best  for  building  up  the  V-rings.  The  unbaked  mica  is 
usually  obtained  in  sheets  0.025  inch  and  0.030  inch  thick. 
Heavier  mica  would  be  difficult  to  bend  and  to  cut  in  shape, 
The  same  sizes  of  hard-baked  mica  with  the  addition  of 
0.02  inch  and  0.035  inch  have  been  found  sufficient  stock  to 
fill  almost  any  size  of  commutator.  In  cutting  up  the  un- 
baked mica  for  the  V-ring  insulation  it  is  safer  first  to  fit  a 
piece  of  paper  around  the  ring  and  then  use  this  as  a  template. 


316 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


Shaping  Mica  End  Rings. — When  it  is  impossible  to  save 
the  old  mica  end  rings,  new  ones  must  be  made.  These  can 
be  made  of  flexible  mica  usually  0.030  inch  thick.  It  is 
best  to  make  the  rings  in  three  pieces  to  the  layer  on  small 
commutators;  three  layers  constituting  a  ring.  When  heated 
slightly,  this  mica  can  be  bent  around  the  sleeve  to  the  desired 
shape.  Trim  the  edges  square  on  each  piece  and  fit  them 
tightly  in  the  bore  of  the  commutator.  The  taper  rings 
should  be  fitted  in  the  same  manner  with  a  slight  space  being 
left  between  abutting  ends. 

Templet  for  Making  Mica  End  Rings. — A  good  shop  method 
for  making  a  template  to  lay  off  mica  end  rings  is  as  follows : 


FIG.  233. — Parts  of  a  medium  sized  commutator. 

<.o)  and  (c)  Mica  end  rings.  (6)  Iron  clamping  V-ring.  (d)  Mica  bushing  sleeve 
for  inside  of  commutator,  (e)  Commutator  bars  and  mica  segments  assembled. 
Sample  bars  and  mica  segments  are  shown  at  (/)  and  (0). 

Mark  off  on  a  piece  of  cardboard  an  arc  of  a  circle,  the  radius  of 
which  is  equal  to  the  diameter  A  in  Fig.  231.  Strike  another 
arc  from  the  same  center  with  the  radius  of  the  dividers  made 
smaller  by  the  distance  C.  This  will  give  a  templet  for  the 
band  D. 

A  templet  for  the  taper  E  can  be  prepared  as  illustrated  in 
the  following  example :  Assume  the  diameter  B  to  equal  seven 
inches.  Then  in  Fig.  231  lay  off  one-half  this  diameter,  or 
inches  from  A  on  the  edge  AB  as  shown  at  0.  If  the 


COMMUTATOR  REPAIRS  317 

taper  is  30  degrees,  take  a  30-  by  60-degree  triangle  and  square 
its  lower  edge  with  the  line  AB,  having  its  apex,  or  point 
touching  the  point  at  0.  Draw  the  line  OC,  which  will  be 
the  radius  of  the  arc  of  a  circle  D,  shown  dotted.  Strike 
another  arc  from  the  same  center  with  the  radius  made  smaller 
by  the  distance  F,  Fig.  231.  This  will  give  a  templet  for  the 
taper  E. 

Micanite  as  a  Commutator  Insulation. — For  use  in  insulating 
commutators,  the  Mica  Insulator  Company,  New  York  City, 
has  developed  a  mica  laying  machine  which  successfully 
constructs  large  sheets  of  micanite  from  thin  mica  laminae 
with  cement  uniformly  applied  between  the  layers.  These 
sheets  are  finished  by  drying  and  baking  under  high  pressure 
after  which  the  sheets  are  packed  through  milling  machine  and 
finished  to  accurate  thickness.  The  company  gives  the  follow- 
ing directions  for  using  micanite  as  a  commutator  insulation. 

If  micanite  segments  for  insulation  between  copper  bars 
of  a  commutator  are  to  be  cut  from  full  size  sheets  of  micanite 
plate,  a  fine  tooth  band  saw  with  moderate  set  should  be  used. 
Bookbinders'  shears  or  foot  power  cutters  are  poor  tools  to 
use  as  the  edge  of  the  micanite  is  likely  to  be  bruised  and  this 
may  result  in  dragging  of  the  copper  over  the  micanite  when 
the  commutator  is  turned  in  a  lathe.  Short  circuits,  or  near 
short-circuits,  are  often  caused  in  this  manner.  It  is  not  neces- 
sary to  coat  the  surface  of  either  the  copper  bars  or  the  mica- 
nite segments  with  shellac  before  assembling  the  commutator. 
After  turning  the  V's  for  the  end  rings  carefully  examine  all 
surfaces  and  scrape  clear  any  places  where  the  lathe  tool  has 
dragged  the  copper.  Before  placing  micanite  rings,  carefully 
remove  all  dust  from  both  commutator  and  rings.  Short- 
circuits  are  sometimes  caused  by  overlooking  these  details. 
After  the  commutator  is  assembled  on  the  shells,  with  the 
rings  in  place  it  should  receive  a  baking  at  a  temperature  of 
about  150°C.  (302°F.).  This  baking  is  for  the  purpose  of 
embedding  the  end  of  the  segments  and  setting  the  cement 
in  the  rings. 

Precautions  when  Tightening  a  Commutator. — It  is  essen- 
tial on  account  of  the  expansion  of  the  copper,  that  the  commu- 
tator is  not  screwed  up  too  tight  before  it  is  baked.  The  best 


318         ARMATURE  WINDING  AND  MOTOR  REPAIR 

results  are  obtained  by  tightening  up  the  commutator  gradu- 
ally after  baking  as  it  cools,  giving  a  final  tightening  when  nearly 
cold.  Many  of  the  troubles  met  with  in  commutators  is  due 
to  excessive  tightening.  In  arch-bound  commutators,  it  is 
important  that  the  assembled  segments  are  not  tightened  too 
hard  in  the  arch  before  the  V  parts  are  turned,  as  the  result- 
ing stresses  from  the  expanding  copper,  distort  the  parts  and 
often  are  the  main  cause  of  high  bars.  Similar  care  should  be 
taken  not  to  exert  more  pressure  than  necessary  on  the  end 
rings  and  whenever  possible  means  should  be  provided  to 
measure  the  pressure.  Cases  have  been  known  where  the 
angles  of  the  clamp  rings  in  small  commutators  have  been 
changed  by  excessive  pressure.  Approximately  1500  Ib.  per 
sq.  in.  on  the  projected  area  of  the  inside  cone  or  taper  rings 
is  a  satisfactory  medium  pressure. 

Making  Micanite  End  Rings. — It  is  often  necessary  for 
the  commutator  builder,  particularly  the  repair  man,  to  make 
up  his  own  rings.  For  that  purpose  a  different  kind  of  plate 
known  as  No.  1  micanite  or  moulding  plate  is  furnished  by 
the  Mica  Insulator  Company.  This  plate  is  made  from  white 
mica  selected  with  same  care  as  for  micanite  commutator  seg- 
ment plate  and  contains  the  proper  proportion  of  cement  to 
produce  satisfactory  moulding  characteristics.  Micanite  is 
not  ductile  and  will  not  stretch  uniformly  like  sheet  metal.  It 
is,  therefore,  necessary  to  cut  the  pattern  for  micanite  com- 
mutator rings  into  the  exact  development  of  the  ring  before 
moulding.  The  pattern  should  be  long  enough  to  allow  for  a 
tapered  splice  at  the  ends.  For  cementing  the  splice,  use  a  high 
grade  shellac  of  about  2  Ib.  to  the  gallon  of  solvent.  When  the 
No.  1  micanite  is  placed  on  a  steam  table  or  metal  plate 
heated  to  about  140°C.  (284°F.),  it  becomes  plastic  and  can 
be  moulded  readily  into  the  desired  shape.  The  micanite 
should  not  remain  on  the  hot  plate  any  longer  than  necessary 
as  the  cement  might  set,  making  it  less  pliable.  It  is  advisable 
to  turn  the  micanite  over  so  as  to  heat  uniformly  from  both 
sides.  After  placing  the  micanite  in  the  mould  or  in  the  com- 
mutator, it  should  be  under  pressure  while  cooling. 

Causes  of  Excessive  Commutator  Wear. — Warren  C.  Kalb 
has  pointed  out  (Power,  March  18,  1919)  that  commutator 


COMMUTATOR  REPAIRS  319 

wear  may  be  caused  by  the  use  of  abrasive  brushes.  It  may 
also  result  from  the  same  mechanical  or  electrical  causes 
that  produce  brush  wear.  Mechanical  conditions  of  this 
nature  are:  Rough  commutator  due  to  poorly  prepared 
surface;  rough  commutator  caused  by  burning;  vibration  from 
any  source  such  as  the  pound  of  a  direct-connected  engine, 
belt  lacing,  improper  mounting,  loose  bearings  or  commutator 
out  of  true;  high  peripheral  speed,  causing  brushes  to  chatter 
or  creating  an  excessive  temperature  at  the  brush  faces  due 
to  friction ;  type  of  brush-holder  or  angle  of  operation,  resulting 
in  the  chattering  of  the  brushes. 

Copper  Used  to  Make  Commutator  Bars. — The  bars  for 
commutators  can  be  made  from  drop  forged,  cold  rolled, 
hard  drawn  or  cast  copper  bars.  The  drop  forged  copper  is 
usually  considered  the  best  and  cast  copper  the  worst  for  bars. 
Probably  most  commutators  are  made  of  cold  rolled  or  hard 
drawn  copper.  Cast  copper  is  poor  on  account  of  the  many 
pin  holes  and  larger  that  are  liable  to  be  in  the  structure  of 
the  bar.  These  show  up  at  every  turning  and  require  filling 
to  prevent  carbon  dust  from  accumulating.  If  they  are  near 
the  edge  of  the  bars  the  carbon  is  liable  to  bridge  the  mica 
between  the  bars  and  cause  a  short-circuit  and  burn  a  hole 
in  two  bars.  This  calls  for  a  patching  of  the  bars  and  the 
insulation  which  adds  a  weak  spot  to  the  commutator  and 
simply  hastens  the  time  when  it  must  be  rebuilt.  A  good 
grade  of  copper  for  bars  is  economical  from  points  of  service 
and  reduction  of  repair  costs. 

However,  when  hard-rolled,  or  drop-forged  copper  is  not 
available  for  the  repair  man,  cast  copper  makes  a  good  substi- 
tute. If  the  castings  are  soft  they  have  a  minimum  conduc- 
tivity of  60  per  cent,  and  are  more  free  from  blow  holes;  also, 
they  wear  from  75  per  cent,  to  90  per  cent,  as  long  as  the  hard- 
drawn  or  drop-forged  copper  bars.  The  castings  are  soft  but 
tough,  and  the  low  conductivity  is  due  to  the  fluxes  used  in 
melting  the  copper.  A  reduction  in  the  amount  of  flux  used 
increases  the  conductivity  of  the  copper,  but  it  also  tends  to 
make  the  metal  more  brittle,  more  porous  and  liable  to  have 
blow  holes  below  the  surface.  The  60  per  cent,  soft-copper 
castings  do  not  need  filling  or  machining  on  the  sides.  They 


320         ARMATURE  WINDING  AND  MOTOR  REPAIR 

can  be  easily  straightened  and  any  projection  flattened  out  on 
a  surface  plate  by  the  aid  of  a  steel  flattener  and  a  hammer. 

Test  for  Oil -saturated  Mica  in  a  Commutator. — When  the 
mica  between  commutator  bars  appears  to  be  saturated  with 
oil  the  following  test  can  be  applied.  Wet  a  rag  or  piece 
of  waste  with  gasoline  and  wipe  the  surface  of  the  commutator 
dry.  Then  with  a  torch  heat  the  commutator  at  the  point 
where  the  mica  appears  to  be  saturated.  If  oil  has  worked 
down  between  the  mica  and  the  bar  it  will  ooze  out  in  small 
bubbles.  In  such  a  case  new  mica  segments  must  be  inserted. 

Blackening  of  a  Commutator  at  Equally  Spaced  Points. — 
This  is  usually  caused  by  an  open  circuit  in  the  armature 
winding.  The  open  circuit  may  be  at  the  commutator  necks 
or  at  the  rear-end  connections  of  the  winding  when  two-piece 
coils  are  used.  When  these  spots  are  noticed  on  a  commutator 
it  should  be  tested  out  for  open  circuits  at  once  before  further 
commutator  troubles  develop. 

UNDERCUTTING  MICA  OF  COMMUTATORS 

The  object  of  undercutting  the  mica  of  commutators  is  to 
clean  out  the  mica  between  the  copper  segments  to  a  depth  of 
about  J^2  to  %2  mcn  s°  that  the  copper  bars  will  wear  evenly 
free  from  the  poor  commutation  that  often  results  when  the 
mica  is  hard  and  does  not  wear  down  evenly  with  the  copper 
segments.  The  undercutting  operation  should  be  done  after 
the  armature  has  been  rewound,  soldered  and  banded.  The 
commutator  should  also  be  trued  up  and  all  excess  solder 
removed  from  the  neck  and  face. 

Tools  for  Undercutting  Mica. — Special  motor-driven  or 
belt-driven  saws  are  available  for  doing  this  work  which  can  be 


FIG.  234. — Hand  tool  for  undercutting  mica  that  can  be  connected  to  a  small 
motor  by  a  flexible  shaft. 

used   when   the  armature  is  centered  in  a  lathe.     The  saws 
are  usually  clamped  on  an  arbor  which  is  mounted  on  a  head 


COMMUTATOR  REPAIRS 


321 


that  moves  on  slide  rails.  By  means  of  a  hand-operated  lever 
or  foot  pedal  controlled  by  the  operator,  the  revolving  saw 
is  carried  over  the  face  of  the  commutator.  A  shaper  equipped 
with  a  special  tool  can  also  be  used.  Frequently  a  milling 
machine  will  be  found  convenient  for  undercutting  the  smaller 
sizes  of  armatures.  A  less  expensive  device  consists  of  a  motor- 
driven  circular  saw  which  is  mounted  in  such  a  way  that  the 
saw  can  be  guided  over  the  commutator  face  by  hand.  Two 
such  motor  driven  tools  are  shown  in  Figs.  234  and  235. 
Size  of  Circular  Saw  Required. — For  the  cutting  tool 
a  circular  saw  or  miller  about  to  1J  inch  in  diameter 


FIG.  235. — Motor  operated  hand  tool  that  can  be  connected  to  a  lighting 
circuit  for  undercutting  the  mica  of  a  commutator. 

with  from  15  to  30  teeth  seems  to  give  the  best  results.  A 
small  diameter  saw  must  be  used  in  order  to  cut  the  slot  to 
the  proper  depth  and  at  the  same  time  not  cut  into  the  neck 
of  the  commutator.  The  saw  should  be  driven  at  approxi- 
mately 1500  rpm.  and  be  about  0.005  inch  thicker  than  the 
mica  in  order  to  remove  the  mica  completely.  If  the  com- 
mutators are  of  large  diameter,  one  or  two  spacers  of  the  same 
thickness  as  the  bars  may  be  used  between  the  saws  and  two 
or  three  slots  cut  at  the  same  time.  The  cutting  edge  of  the 
saw  should  revolve  in  a  direction  toward  the  operator  while 
cutting  the  mica.  When  the  hand-operated  motor-driven 
21 


322         ARMATURE  WINDING  AND  MOTOR  REPAIR 

tools  are  used  they  should  be  drawn  toward  the  operator, 
in  order  to  properly  guide  the  tool. 

It  is  advisable  to  have  a  jet  of  compressed  air  or  a  fan  so 
located  that  the  particles  of  mica  and  copper  will  be  blown 
away  from  the  armature  to  prevent  this  material  falling  in 
behind  the  commutator  and  at  the  same  time  make  it  easy 
for  the  operator  to  see  the  slot  when  the  saw  is  throwing  the 
particles  toward  him  on  the  face  of  the  commutator. 

Finishing  Slots  and  Commutator  Surface  after  Undercut- 
ting.— After  the  slots  are  sawed  it  is  advisable  to  go  over  them 
with  a  sharp  hand  tool  to  remove  remaining  particles  of  mica 
and  thin  strips  along  the  edges  of  the  slot.  This  can  be  done 
with  a  sharp  knife,  or  V-shaped  tool,  which  can  also  be  used  to 
slightly  bevel  the  sharp  edges  of  each  bar  and  remove  all 
burrs.  Several  hand  tools  that  can  be  used  for  undercutting 
mica  when  motor  operated  tools  are  now  available  are  described 
in  the  following  paragraphs. 

After  the  undercutting  operation  has  been  completed  the 
commutator  should  be  stoned  and  polished  with  fine  sandpaper 
to  remove  all  burrs  of  copper. 

Brushes  for  Use  on  Undercut  Commutators. — The  removal 
of  mica  permits  the  use  of  brushes  without  abrasive  qualities 
and  has  resulted  in  the  development  of  very  low  friction 
brushes.  In  fact  some  of  the  very  lowest  friction  brushes 
manufactured  are  hard  and  do  not  contain  any  graphite. 
Such  brushes  give  to  a  slotted  commutator  the  brown  gloss 
that  shows  perfect  operation.  The  life  of  a  hard  non-abra- 
sive brush  is  several  times  that  of  a  graphite  brush.  Some 
criticism  has  been  made  of  slotted  commutators  which  has 
been  traced  to  the  fact  that  the  brushes  used  were  not  suitable. 
Often  when  the  same  brushes  are  used  on  an  undercut  com- 
mutator that  were  used  before,  the  commutator  will  be  ridged 
and  worn  rapidly  and  the  slots  filled  with  copper  dust  with 
short-circuits  and  burned  out  coils  the  result.  The  fault  in 
such  cases  is  in  the  brush  not  in  the  undercutting  process. 

It  should  also  be  remembered  that  the  brush  tension  on 
an  undercut  commutator  can  often  be  much  lower  than  before. 
Brush  authorities  recommend  lj^  to  2J£  pounds  per  square 
inch  of  cross-section  on  stationary  motors,  from  3  to  5  pounds 


COMMUTATOR  REPAIRS 


323 


per  square  inch  for  crane  motors  and  from  4  to  8  pounds  per 
square  inch  on  railway  motors  when  the  commutators  are 
undercut. 

Another  factor  in  good  operation  of  undercut  commutators 
is  cleaning  at  regular  intervals.  If  the  motor  is  operated  in  a 
place  where  the  accumulated  dust  and  dirt  is  dry,  it  can  be  re- 
moved with  a  blast  of  compressed  air.  Even  in  such  cases 
the  slots  should  be  scraped  occasionally.  A  lubricant  should 
never  be  used  on  an  undercut  commutator  nor  any  other  when 
operating  conditions  are  correct. 

Hand  Tools  for  Undercutting  Mica  of  Commutators. — An 
easily  made  hand  tool  for  undercutting  the  mica  of  com- 


FIG.  236. 


-Short  saw  blade  clamped  in  a  vulcanized  fiber  holder  for  use  in 
undercutting  mica. 


mutators  is  shown  in  Fig.  236  as  devised  by  William  H.  Watson 
(Power,  Oct.  1,  1918).  The  slot  in  the  holder  is  made  as  deep 
or  deeper  than  the  width  of  the  hacksaw  blade,  with  small  bolts 
to  clamp  the  blades  at  each  end.  Any  depth  of  cut  desired  can 
be  made  by  adjusting  the  blade  in  the  holder,  but  there  sel- 
dom is  occasion  to  move  the  blade.  The  handle  part  is  cut 
away  just  enough  to  allow  the  fingers  to  pass  over  the  commu- 
tator without  rubbing.  In  undercutting  commutators  care 
must  be  taken  that  no  rough  edges  are  left  after  the  mica  is 
cut,  but  it  is  sometimes  hard  to  avoid  this,  especially  if  it  is  an 
old  commutator.  These  rough  corners  can  be  smoothed,  after 


324         ARMATURE  WINDING  AND  MOTOR  REPAIR 

the  mica  has  been  undercut,  with  a  V-shaped  tool  made  of 
hardwood  or  vulcanized  fiber,  run  between  the  segments  as 
shown  at  A,  and  it  does  not  spoil  the  bar.  The  front  corner 
of  the  undercutter,  which  is  made  of  fiber,  can  be  used  to 
smooth  these  raw  edges. 

Another  hand  tool  recommended  by  T.  M.  Sterling  (Power, 
Nov.  5,  1918)  for  undercutting  mica  and  as  useful  in  other 
ways  by  the  repairman,  is  illustrated  in  Fig.  237.  In  the 
illustration  A  represents  a  steel  straight-edge  about  ^  inch 
thick  and  slightly  longer  than  the  length  of  the  commutator 
segments  and  square  at  one  end  to  butt  up  against  the  head  of 
the  commutator.  B  is  the  undercutting  tool,  about  12  inches 


FIG.  337. — Hand  tool  for  undercutting  commutator  mica. 

long,  made  from  a  piece  of  %Q  hexagonal  tool  steel  forged  flat 
for  about  one-fourth  of  its  length  to  a  thickness  at  point  D 
equal  to  the  thickness  of  the  mica  between  the  segments.  The 
"half-heart"  shaped  lobe  C  affords  a  bearing  place  for  the 
fingers  in  using  the  tool. 

When  using  this  tool  lay  the  straight-edge  on  the  commu- 
tator with  its  edge  in  line  with  the  mica  between  the  segments 
and  the  square  end  against  the  head  of  the  commutator. 
Hold  it  firmly  with  the  left  hand.  Then,  holding  tool  B  in 
the  right  hand,  draw  the  point  D  along  the  mica  the  same  as 
drawing  a  pencil  along  a  ruler  in  ruling  paper.  This  operation 
will  start  a  nice  groove  in  the  mica  without  burring  the  edges 
of  the  copper  on  either  side.  After  one  or  two  passes  the  tool 
can  be  turned  endwise  with  point  D  down  and  the  mica  taken 
to  any  desired  depth  or  a  saw  blade  may  be  used  to  finish  the 


COMMUTATOR  REPAIRS  325 

groove.  The  disadvantage  of  the  saw  blade  is  that  the  set 
of  the  teeth  burr  the  copper  more  or  less,  while  with  this  tool 
all  chance  of  a  burr  is  eliminated. 

The  tool  will  also  be  found  handy  in  raising  coil  leads  out 
of  the  slots  in  the  head  of  a  commutator  when  unsoldering 
them  preparatory  to  removing  the  commutator  or  making 
coil  repairs.  After  heating  the  ends  of  the  coils  until  the  solder 
starts  to  melt,  drive  the  point  of  the  tool  into  the  slot  under 
the  ends  of  the  wires,  with  lobe  C  down,  for  a  fulcrum,  on  the 
bottom  of  the  slot.  The  ends  of  the  coils  can  then  be  easily 
raised  by  a  downward  pressure  of  the  hand.  Square  edge 
E  can  be  used  as  a  scraper  for  taking  the  surplus  hot  solder 
off  the  ends  of  the  wires  as  they  are  raised. 


CHAPTER  XIII 

ADJUSTING    BRUSHES    AND    CORRECTING    BRUSH 

TROUBLES 

Many  of  the  troubles  which  are  charged  to  brushes  of  motors 
and  generators  can  be  traced  to  improper  application  and 
adjustment  or  to  other  defects  of  the  machine  that  show 
up  in  sparking  at  the  commutator.  A  careful  selection  of 
brushes  is  important  but  no  more  so  than  carefulness  in  ad- 
justment and  frequent  inspection  in  operation,  for  proper 
care  of  brushes  and  brush  rigging  results  in  good  commutation 
and  prolonged  life  of  both  brushes  and  commutator.  The 
repairman  should,  therefore,  give  due  consideration  to  brush 
adjustment  when  overhauling  a  machine. 

Fitting  or  Grinding-in  Brushes  (Instruction  Book,  Westing- 
house  Electric  &  Mfg.  Co.). — When  it  becomes  necessary 
to  install  a  new  set  of  brushes,  sand  paper  or  garnet  paper 
may  be  used  that  is  long  enough  to  go  around  the  commutator. 
It  should  have  a  lap  of  several  inches  and  be  so  mounted  on  the 
commutator  as  to  preclude  the  lapped  end  butting  against  the 
brushes  when  the  machine  is  rotated  for  grinding.  With 
many  commutators  the  friction  between  the  commutator  bars 
and  sandpaper  (if  the  paper  is  taut)  will  suffice  to  keep  the 
paper  from  slipping,  especially  if,  when  starting,  the  paper  is 
given  a  pull  in  the  direction  of  rotation  by  the  operator. 
If  the  paper  persists  in  slipping  use  a  little  glue  to  stick  the 
under  end  of  the  paper  at  the  lap  to  the  commutator.  All 
traces  of  the  glue  must  be  removed  from  the  bars  before 
putting  the  machine  into  service. 

A  second  method  is  to  remove  the  middle  brush  on  each  arm 
and  bind  the  paper  to  the  commutator  by  running  tape  or 
string  entirely  around  the  periphery.  If  necessary,  also  bind 
the  paper  at  the  inner  and  outer  ends  of  the  commutator. 
After  the  paper  is  anchored  the  commutator  may  be  rotated 

326 


ADJUSTING  BRUSHES  AND  CORRECTING  TROUBLES       327 

by  hand,  or  in  any  convenient  way.  Great  care  must  be 
exercised  in  " grinding  in"  brushes  in  this  way,  as  the  cutting 
is  very  rapid,  especially  with  soft  brushes,  and  much  of  the  life 
of  the  brush  may  be  ground  away  in  a  very  few  revolutions. 
If  after  the  brushes  have  been  surfaced  in  the  above  manner, 


FIG.  238. — Grinding-in  brushes  of  a  direct-current  generator  with  a  strip  of 

sandpaper. 

the  trailing  edge  shows  a  poor  seat  on  account  of  having  had 
to  mount  the  ridge  due  to  the  lap  in  the  paper,  a  final  surfacing 
should  be  done  with  very  fine  sandpaper  by  hand.  The 
foregoing  method  is  particularly  desirable  when  a  machine 
has  very  hard  brushes,  or  a  large  number  of  soft  brushes. 


328          ARMATURE  WINDING  AND  MOTOR  REPAIR 

It  is  a  great  time-saver  after  the  knack  of  applying  and  anchor- 
ing the  paper  is  understood. 

With  slip  rings,  if  carbon  or  graphite  brushes  are  used,  the 
same  general  scheme  is  applicable,  but  if  metal  graphite 
brushes  are  used,  emery  cloth,  or  its  equivalent,  is  preferable. 

There  are  those  who  advocate  and  those  who  practice 
pulling  the  grinding  paper  in  the  direction  of  rotation  in 
order  to  more  accurately  surface  the  brush.  This  seems 
logical  enough  on  first  thought,  but  inasmuch  as  the  com- 
mutator is  not  in  motion  when  the  brushes  are  surfaced  in 
this  manner  there  is  no  assurance  that  the  brush  will  bear 
the  same  relation  to  its  holder  (and  therefore  to  the  com- 
mutator) when  the  commutator  is  rotating  and,  consequently, 
no  assurance  that  the  contact  will  remain  fixed.  This  perhaps 
explains  why  brushes  frequently  show  perfect  contact  when 
idle,  but  poor  contact  when  in  service.  This  method  of 
fitting  the  brushes,  therefore,  is  not  necessarily  more  depend- 
able than  other  methods,  though  in  some  instances  it  can 
be  recommended. 

After  the  brushes  have  been  fitted  all  dust  should  be  care- 
fully blown  out  of  the  commutator  by  compressed  air.  The 
pressure  should  be  about,  50  to  80  pounds  per  square  inch  used 
with  a  J^-inch  nozzle.  A  higher  pressure  may  injure  insula- 
tion and  is  not  necessary. 

It  is  important  that  the  brushholders,  whether  for  direct 
or  alternating-current  use,  be  neither  too  close  to  nor  too  far 
from  the  commutator  or  slip  rings.  A  suitable  distance  is 
from  three-sixteenths  to  one-quarter  of  an  inch. 

If  the  brushes  are  copper-coated,  the  coating  should  be 
not  allowed  to  come  in  contact  with  the  commutator  or  slip 
rings.  This  means  that  as  the  brushes  wear  the  copper  coating 
should  be  scraped  back  and  not  allowed  to  extend  below  the 
brushholder  box.  With  proper  shunts  there  should  be  no 
need  for  copper  coating  from  an  electrical  standpoint,  although 
for  mechanical  reasons  it  may  be  desirable  as  a  protection 
for  soft  or  structurally  weak  brushes. 

If  the  toe  of  the  brush  is  very  sharp  it  is  good  practice  to 
"nose  off"  the  knife  edge.  This  reduces  breakage  and  mili- 
tates against  gouging  into  the  commutator. 


ADJUSTING  BRUSHES  AND  CORRECTING  TROUBLES       329 

Adjustment  of  Brushholders  (Instruction  Book,  Westing- 
house  Electric  &  Mfg.  Co.). — If  the  commutator  or  slip-ring 
speed  is  low,  a  holder  having  a  sluggish  spring  and  slow  action 
will  give  entirely  satisfactory  results  but  such  a  holder  on  a 
high-speed  machine  may  fail  miserably.  High  speeds  require 
quickness  of  action  on  the  part  of  the  brush.  This  means 
a  sensitive  quick-acting  spring  and  a  holder  designed  to  aid 
the  spring.  A  high-speed  holder,  therefore,  may  give  excellent 
results  on  low-speed  machines,  but  a  low-speed  holder  is  not 
likely  to  approach  even  fair  results  on  high-speed  machines. 
This  is  a  point  a  repairman  must  bear  in  mind  when  changing 
the  speeds  of  direct-current  machines. 

The  box,  or  part  holding  the  brush,  should  be  as  nearly  a 
full  box  as  possible.  Whether  the  rotation  is  against  the  toe 
or  against  the  heel,  both  the  toe  and  heel  sides  of  the  holders 
should  have  approximately  equal  areas  for  the  support  of  the 
brushes.  It  is  obvious  that  the  shorter  the  box,  the  greater 
will  be  the  shifting  movement  of  the  brush  on  the  commutator 
due  to  looseness  in  the  holder;  and  the  deeper  the  box,  the 
less  will  be  the  shifting.  From  this  it  follows  that  the  boxes 
should  not  be  too  short,  nor  should  the  brushes  be  too  loose 
in  the  boxes. 

It  is  an  established  fact  that  with  even  very  slight  play  in  the 
holder  the  changing  brush  friction  due  to  load,  temperatures, 
etc.,  will  result  in  a  changing  relation  between  the  brush  and 
the  commutator,  and  thereby  cause  changing  and  misleading 
brush  drops  from  day  to  day.  This  accounts  for  the  difficulty 
in  verifying,  on  different  days,  brush  drops,  although  the  loads 
may  be  identical.  The  neutral  does  not  change,  but  the 
relation  of  brush  contact  to  commutator  does  change,  and 
the  brush  drops  vary  accordingly.  Another  explanation  for 
the  changes  that  take  place  in  brush  drops  is  that  when 
brushes  have  been  freshly  " sand-papered "  or  " ground  in" 
the  surfaces  have  unglazed  or  soft  faces,  the  brush  drops 
being  influenced  by  the  porous  condition  of  the  faces,  and  the 
lubricating  values  of  the  particles  liberated  during  the  time  the 
brush  face  is  seating  itself  to  the  commutator.  During  this 
period  the  commutation,  with  certain  kinds  of  graphitized 
brushes,  may  be  much  better  than  after  the  glaze  or  permanent 


330         ARMATURE  WINDING  AND  MOTOR  REPAIR 

surface  forms  on  the  brush.  Drops  taken  during  this  time  may 
not  be  the  same  as  drops  taken  after  the  brush  is  "faced  up" 
and  may  prove  seriously  misleading.  Definite  or  reasonably 
permanent  drop  values  are,  as  a  rule,  to  be  had  only  after 
the  brush  faces  have  reached  a  fixed  condition. 

From  the  foregoing  it  is  obvious  that  brushes  should  fit 
snugly,  but  not  tightly,  in  the  holders;  also  that  there  are 
certain  relative  dimensions  which  are  preferable  between 
the  brushes  and  holders  and  which  must  not  be  ignored  if  the 
best  results  are  to  be  obtained.  For  instance,  the  side  of  the 
holder  in  the  direction  of  rotation  should  not  have  less  area 
than  the  lagging  side.  In  fact,  the  side  against  which  the 
brush  presses  should  have  preferably  a  greater  area  than 
the  opposite  side — certainly  not  less  area.  With  a  limited 
area  to  oppose  the  brush,  movement  of  the  brush  in  the  holder 
due  to  looseness,  oscillation  and  imperfect  relations  between 
the  brush  and  commutator,  accentuates  the  change  in  the 
angle  of  contact,  and  results  in  poor  commutation,  as  well  as 
in  mechanical  and  electrical  damage  to  that  part  of  the  brush 
in  contact  with  the  holder. 

If  a  brush  is  thin,  its  length  and  holder  contact  should  be 
greater  than  if  it  is  thick,  for  the  reason  that  the  thinner  the 
contact  area  on  the  commutator,  the  less  stable  will  be  the 
brush  on  the  commutator,  and  the  greater  will  be  the  need 
of  support  from  its  holder.  To  illustrate : — If  the  brush  con- 
tact is  a  knife  edge,  serious  chattering  may  result  and  there 
may  be  a  maximum  of  instability,  but  if  the  knife  edge  be 
gradually  removed  the  brush  will  become  increasingly  stable 
as  the  thickness  of  the  brush  contact  increases,  and  if  the 
brush  angle  is  correct  the  maximum  stability  will  be  reached 
when  the  brush  has  its  greatest  thickness  in  contact  with  the 
commutator.  It  stands  to  reason,  therefore,  that  the  greater 
the  area  due  to  thickness,  the  more  stable  will  be  the  brush 
on  the  commutator,  regardless  of  the  holder,  and  that  as  a 
consequence  a  holder  for  a  thick  brush  might  not  prove  at  all 
suitable  for  a  thin  brush,  the  speeds  being  equal.  If,  however, 
there  is  the  same  amount  of  lost  motion  in  the  holders  for  a 
given  length  of  thin  brush  and  thick  brush,  and  the  brush 
angle  is  such  that  the  thick  brush  moves  its  maximum  in  the 


ADJUSTING  BRUSHES  AND  CORRECTING  TROUBLES       331 

holder,  then  the  thick  brush  will  show  a  greater  change  in  its 
relation  to  the  commutator  than  the  thin  brush.  The  thick 
brush,  however,  will  not  be  so  likely  to  shift  its  maximum  on 
account  of  its  bearing  surface  being  greater  than  the  bearing 
surface  of  the  thin  brush.  In  other  words,  it  rests  on  a 
greater  area. 

Causes  of  Rapid  Brush  Wear. — Warren  C.  Kalb  (Power, 
March  18,  1918)  has  found  that  the  following  electrical  con- 
ditions cause  rapid  brush  wear:  Sparking,  from  any  cause 
whatever;  glowing  of  brushes;  pitting  of  brush  faces. 

Glowing  results  from  excessive  current  density  and  may 
be  local  or  may  cover  the  entire  contact  end  of  the  brush. 
Common  causes  are  unequal  collection  of  current  by  different 
brushes  of  the  same  polarity  and  very  heavy  short-circuit 
currents  in  the  coils  undergoing  commutation,  which,  added 
to  the  load  current,  bring  the  temperature  of  the  carbon 
at  the  face  of  the  brush  up  to  the  glowing  point. 

Pitting  may  result  from  glowing  in  small  spots  on  the  brush 
face.  It  is  also  caused  at  times  by  particles  of  copper  becom- 
ing attached  to  the  brush  face.  This  causes  a  heavy  current 
to  localize  in  a  small  area,  disintegrating  the  carbon  and 
forming  a  small  crater  in  which  the  copper  embeds  itself. 

The  increased  brush  wear  caused  by  increase  in  current 
density  is  due  to  the  higher  temperature  so  created  at  the 
brush  face  and  the  consequent  more  rapid  disintegration  of 
the  carbon  at  that  point.  Any  factor  tending  to  further 
increase  the  temperature  at  the  brush  face  will  add  to  the 
rapidity  of  wear.  Some  of  the  things  having  this  effect  are: 
High  coefficient  of  friction ;  higher  contact  drop  than  is  needed 
for  commutation,  especially  on  machines  of  low  voltage  and 
high  current  capacity;  contact  drop  too  low  for  sparking 
voltage,  permitting  heavy  currents  to  flow  in  the  short- 
circuited  coil;  lack  of  carrying  capacity  or  very  high  current 
density. 

Higher  current  densities  are  permissible  where  there  is  no 
sparking  voltage  than  where  there  is.  This  is  because  the 
heating  effect  of  commutation  current  is  absent  and  higher 
load  currents  can  be  applied  before  the  same  temperature 
rise  is  attained. 


332         ARMATURE  WINDING  AND  MOTOR  REPAIR 

Average  volts  per  commutator  segment  may  be  as  high  as 
15  to  20  volts  without  difficulty  being  encountered.  Or 
the  reactance  voltage  may  be  that  high  and  still  be  neutralized 
by  interpole  flux  or  fringing  field  within  sufficiently  close 
limits  to  secure  good  commutation.  But  the  sparking 
voltage — that  is,  the  resultant  between  the  reactance  voltage 
and  the  other  electromotive  forces  generated  within  the  coil 
undergoing  commutation — should  be  less  than  the  contact 
drop  of  positive  plus  negative  brush  to  attain  perfect  com- 
mutation. Inasmuch  as  3  volts  is  about  the  highest  brush- 
contact  drop  obtainable,  it  will  be  seen  that  the  sparking 
voltages  mentioned  would  not  be  neutralized  within  12  to  17 
volts.  Such  a  voltage  would  set  up  excessive  currents  in  the 
short-circuited  coils  and  result  in  rapid  burning  away  of  the 
brush  faces  even  with  no-load  current  whatever  carried  by 
the  machine. 

Methods  for  Locating  the  Electrical  Neutral  in  Setting 
Brushes. — The  following  methods  have  been  suggested  by 


i: 


V 
FIG.  239. — Pilot-brush  method  of  locating  brushes  on  the  electrical  neutral. 

T.  F.  Barton  (Power,  July  16,  1918)  for  setting  brushes  on 
the  electrical  neutral  in  direct-current  machines. 

With  the  machine  running  at  no  load,  normal  speed  and 
voltage,  shift  the  brushes  until  a  low-reading  voltmeter  shows 
no  deflection  when  connected  to  points  just  inside  the  heel 


ADJUSTING  BRUSHES  AND  CORRECTING  TROUBLES       333 

and  toe  of  a  pilot  brush.  The  pilot  brush  must  be  the  full 
size  of  the  brushes  used  on  the  machine  and  can  be  made  of 
fiber  or  wood  with  holes  drilled  through  it  to  allow  contact 
on  the  commutator  at  the  desired  point,  which  should  be 
at  the  center  of  two  adjacent  commutator  bars  as  indicated 
in  Fig.  239. 

Operate  the  machine  as  a  shunt-wound  commutating-pole 
motor,  checking  the  speed  in  both  directions  of  rotation, 
holding  the  same  value  of  armature  voltage  and  shunt-field 
current  in  each  case.  The  brushes  are  on  neutral  when  the 
speed  is  the  same  in  each  direction  of  rotation. 

It  is  preferable  to  locate  the  brushes  on  the  electrical  neutral 
and  make  the  adjustments  on  the  commutating-  and  series- 
field  windings  to  give  the  desired  results.  Brush  shift  can  be 
used,  provided  the  final  brush  position  is  in  a  satisfactory 
commutating  zone.  The  capacity  of  a  machine  is  not  re- 
duced by  shifting  the  brushes  if  the  rated  voltage  and  current 
can  be  successfully  obtained. 

Angle  at  which  Brush  is  Set. — When  machines  are  not 
subject  to  reversals,  the  proper  angle  for  the  brush  will  depend 
upon  whether  the  rotation  is  against  the  toe  of  the  brush  or 
against  the  heel.  It  also  depends  upon  the  friction  coefficient 
of  the  brush.  If  set  against  the  toe,  the  angle  should  approxi- 
mate 35  degrees.  Above  35  degrees  the  toe  becomes  very 
sharp  and  mechanically  weak.  If  set  against  the  heel,  the 
angle  may  be  from  12  degrees  to  25  degrees.  In  instances 
where  the  brush  touches  more  bars  than  required,  the  sharp 
toe  can  be  beveled  off  thereby  reducing  the  brush  contact 
and  the  short-circuiting  current  under  the  brush.  This  also 
increases  the  mechanical  strength  of  the  brush.  Whether 
running  against  toe  or  heel,  it  is  good  practice  to  round  off  the 
sharp  or  knife  edge  of  the  toe.  This  reduces  the  possibility 
of  the  knife-like  edges  digging  into  the  commutator  in  case 
the  brushes  jam  or  become  tight  in  their  holders  and  at  the 
same  time  protects  against  brush  breakage. 

The  friction  coefficient  of  a  brush  is  a  variable  one,  influenced 
by  the  characteristics  of  the  brush,  by  the  glazing  of  the 
contact  face  of  the  brush,  the  varying  brush  temperature  due  to 
load  and  spring  pressure  and  the  condition  of  the  commutator. 


334         ARMATURE  WINDING  AND  MOTOR  REPAIR 

whether  it  is  smooth  or  undercut,  hot  or  cold,  in  good  or  poor 
mechanical  condition.  The  poor  mechanical  condition  of  the 
commutator  can  be  eliminated  by  proper  attention.  The 
other  influences  are  permanent  except  as  they  may  be  modified 
by  finding  the  most  suitable  brush  angle. 

Checking  Brush  Setting. — To  check  the  spacing  of  brushes, 
place  a  strip  of  paper  completely  around  the  commutator  under 
the  brushes  and  mark  the  position  of  each  set.  The  strip  can 
then  be  removed  and  the  distance  between  marks  measured. 
If  the  distance  varies  the  brush  holders  should  be  moved  into 
the  proper  position.  The  degree  of  accuracy  with  which 
brushes  should  be  set  depend  upon  the  type  of  machine. 
With  interpole  machines,  the  spacing  should  be  more  accurate 
than  with  other  types  of  machines.  The  brushes  on  any  given 
stud  should  be  staggered  with  respect  to  those  on  adjacent 
studs  so  that  the  entire  commutator  will  be  covered  by  the 
brushes  except  a  slight  space  at  each  end.  This  prevents 
grooves  wearing  in  the  commutator. 

Brush  Pressure. — This  depends  upon  the  character  of  the 
brush,  the  commutator  speed  in  feet  per  minute,  and  the 
mechanical  condition  of  the  commutator.  It  varies  from  two 
to  five  pounds  per  square  inch,  the  pressures  generally  used 
being  from  2.5  to  3.5  pounds  on  commutators  and  from  three  to 
five  pounds  on  slip  rings.  If  there  is  a  tendency  for  the  com- 
mutator to  burn,  this  can  at  times  be  corrected  by  increasing 
the  pressure,  and  thereby  increasing  the  abrasive  effect  of  the 
brush  sufficiently  to  scour  out  the  burning  and  maintain  a 
polished  surface.  When  the  mica  is  not  undercut  the  brush 
should  have  sufficient  abrasive  effect  not  only  to  scour  out 
possible  burning  of  the  commutator  bars  but  also  to  wear  and 
keep  the  mica  flush  with  the  commutator  bars. 

Common  Brush  Terms. — The  following  terms  are  frequently 
used  in  describing  brush  characteristics  and  their  performance. 

Contact  Drop. — This  refers  to  the  drop  in  voltage  between 
the  brush  and  the  commutator.  It  differs  with  different 
materials,  different  brush  pressures  and  different  loads,  being 
between  one  and  two  volts.  For  a  given  material  and  given 
load  the  contact  drop  decreases  with  increased  pressure  and 
increases  with  the  load.  It  varies  little  with  changes  in  speed 


ADJUSTING  BRUSHES  AND  CORRECTING  TROUBLES       335 

above  2500  feet  per  minute  and  but  little  with  temperature 
changes  under  normal  conditions. 

Brush  Friction. — The  coefficient  of  friction  depends  upon 
the  brush  material,  brush  angle  and  commutator  speed.  With 
a  given  material  and  brush  angle,  the  coefficient  of  friction 
increases  with  load  and  brush  pressure  and  in  general  decreases 
with  increased  commutator  speed.  Increased  temperatures,  in 
actual  practice,  justify  the  conclusion  that  the  friction  coef- 
ficient increases  with  increased  temperatures. 

Specific  Resistance. — The  ohmic  resistance  of  a  cube  having 
one-inch  sides. 

Current  Density  or  Carrying  Capacity. — This  is  based  on  the 
maximum  continuous  current  capacity  per  square  inch,  with- 
out glowing,  honeycombing,  undue  heating  or  sparking. 

Glowing. — A  brush  is  said  to  "glow"  when  it  becomes  red  or 
incandescent  in  spots  in  proximity  to  the  commutator.  This 
is  due  to  the  short-circuiting  current  under  the  brush,  or  to  the 
short-circuiting  current  in  combination  with  the  working  or 
load  current.  It  may  also  be  due  to  a  lack  of  homogeneity 
of  the  brush  (hard  or  soft  spots  of  the  same  or  different  mate- 
rials having  characteristics  foreign  to  the  material  in  the  body 
of  the  brush);  to  incorrect  brush  position;  to  improper  brush 
selection;  to  selective  commutation;  to  the  brush  covering  too 
many  bars;  to  high  mica;  to  the  machine  having  inherently 
poor  commutating  characteristics  due  to  improperly  shaped 
or  spaced  main  poles  or  commutating  poles;  to  bad  commutat- 
ing pole  adjustments,  or  to  a  poor  distribution  of  the  armature 
windings.  In  the  event  the  glowing  can  not  be  stopped  by 
correcting  such  of  the  brush  or  mechanical  troubles  as  may 
exist;  or  by  shifting  the  brushes,  or  readjusting  the  commutat- 
ing poles,  or  both ;  a  brush  having  higher  current  density  values 
and  less  susceptible  to  the  effects  of  high  voltages  between  the 
commutator  bars  should  be  installed.  If  a  suitable  brush 
can  not  be  had,  the  design  of  the  machine  itself  may  be  subject 
to  modification. 

Honeycombing. — When  brushes  gradually  burn  away,  form- 
ing small  craters  in  their  faces,  they  are  said  to  "  honey  comb." 
This  is  due  to  continuous  sparking  of  a  more  or  less  hidden 
nature  and  may  be  of  either  a  slow  or  rapid  nature.  If  the 


336          ARMATURE  WINDING  AND  MOTOR  REPAIR 

growth  is  slow  it  is  sometimes  possible  to  correct  it  by  increas- 
ing the  brush  pressure,  thereby  decreasing  the  contact  drop 
and  the  contact  arcing,  and  at  the  same  time  increasing  the 
abrasive  effect  of  the  brush  and  grinding  away  the  minute 
craters  as  they  form.  Honeycombing  is  due  to  the  same 
causes,  on  a  reduced  scale,  as  glowing,  and  at  times  both  may 
take  place  simultaneously  on  the  same  machine.  Honey- 
combing has  been  traced  directly  to  high  mica,  in  combination 
with  loose  commutators;  also  to  improperly  spaced  main 
poles ;  and  unequal  air  gaps;  and  in  non-commutating-pole 
machines,  to  an  abnormal  shifting  or  distorting  of  the  mag- 
netic flux.  The  trouble  is  most  frequently  corrected  by  sub- 
stituting a  more  highly  refractory  brush  possessing  abrasive 
characteristics. 

Hardness. — Brushes  have  widely  varying  physical  densities 
— some  being  very  hard,  others  very  soft.  Very  hard  brushes 
usually  carry  a  large  amount  of  abrasive  and  have  a  low  current 
density  or  carrying  capacity.  The  softer  brushes  are  more 
highly  graphite,  carry  abrasive  to  a  limited  extent  only  and 
have  a  high  current  density.  The  hardness  or  scleroscope 
reading  is,  therefore,  indicative  to  a  limited  degree  of  the  char- 
acter of  the  brush. 

Brush  Inertia. — This  has  to  do  with  the  weight  of  the  brush. 
The  lighter  the  brush,  the  more  readily  will  it  follow  the  irregu- 
larities of  the  rotating  element,  and  the  more  promptly  will 
it  respond  to  a  given  spring  pressure.  Again,  the  wearing  of 
the  commutator  may  not  prove  so  great.  If  a  light  and  a 
heavy  brush  prove  equally  satisfactory  electrically  on  a  ma- 
chine, the  lighter  brush  is  preferable.  If  the  materials  from 
which  brushes  are  made  are  heavy,  it  is  advisable  to  have  an 
increased  number  of  small  brushes,  rather  than  a  limited  num- 
ber of  large  brushes;  also  the  spring  pressure  should  be  in 
excess  of  the  pressure  on  lighter  brushes  having  the  same 
dimensions,  and  used  for  the  same  service. 

Refractory. — Any  material  which  resists  the  ordinary  meth- 
ods of  reduction  is  said  to  be  refractory.  A  brush,  therefore, 
which  resists  wholly,  or  to  a  marked  degree,  high  tempera- 
tures, such  as  the  heat  generated  by  an  electric  arc,  is  said  to 
be  highly  refractory. 


ADJUSTING  BRUSHES  AND  CORRECTING  TROUBLES       337 

Peripheral  Speed. — This  is  the  speed  in  feet  per  minute  of 
the  commutator  or  slip  ring.  Peripheral  speeds  which  vary 
greatly  require  brushes  having  different  characteristics.  Gen- 
erally speaking,  carbon  brushes  are  not  as  suitable  for  high 
peripheral  speeds  as  graphite  brushes. 

Procedure  for  Locating  Causes  of  Brush  Trouble. — Spark- 
ing at  the  brushes  of  a  motor  or  generator  can  usually  be  traced 
to  one  or  more  of  the  following  causes. 

1.  Too  low  brush  pressure. 

2.  Incorrect  spacing  of  brushes. 

3.  Unequal  air-gaps  or  defective  fields. 

4.  Brushes  not  operating  on  electrical  neutral. 

5.  Incorrect  thickness  of  brushes. 

6.  Using  brushes  of  wrong  characteristics. 

The  above  causes  are  given  in  the  order  in  which  they  should 
be  checked  up  in  the  machine.  All  but  two  of  these  defects 
may  cause  abnormal  short-circuit  currents  under  the  face  of 
each  brush  or  between  two  or  more  studs  of  the  same  polarity. 
This  point  should  be  kept  in  mind  when  searching  out  the 
trouble.  The  procedure  for  investigating  each  of  the  causes 
has  been  outlined  by  E.  H.  Martindale  of  the  National  Carbon 
Company  as  follows: 

Before  starting  an  investigation,  eliminate  the  question  of 
high  mica.  If  the  commutator  runs  nearly  true,  stone  it 
enough  to  grind  the  mica  even  with  the  bars.  If,  however,  the 
commutator  is  not  true,  or  there  are  flat  bars,  turn  or  grind  the 
commutator  before  looking  further  for  the  seat  of  the  trouble. 

Too  Low  Brush  Pressure. — Low  pressure  will  cause  poor  con- 
tact between  the  brushes  and  the  commutator,  and  force  the 
current  to  pass  from  one  to  the  other  through  a  small  arc. 
This  will  burn  both  the  brushes  and  commutator  and  produce 
high  mica.  The  heat  of  the  arcs  between  brushes  and  commu- 
tator will  also  increase  the  temperature  rise.  Further,  poor 
pressure  produces  a  high  contact  drop,  which  will  also  heat 
the  commutator  and  brushes.  This  high  contact  drop  and 
arcing  caused  by  too  low  pressure  frequently  heats  the  brushes 
to  a  red  heat,  or,  as  we  say,  "to  the  glowing  point, "  which  is 
always  accompanied  by  pitting  or  disintegration  of  the  faces 

of  the  brushes.     This  again  reduces  the  available  contact  area 
22 


338         ARMATURE  WINDING  AND  MOTOR  REPAIR 

and  increases  the  current  density  through  the  balance  of  the 
brush.  The  bad  effects  of  too  low  brush  pressure  are  greatly 
aggravated  when  the  commutator  is  slightly  elliptical  or  runs 
unbalanced. 

Whether  the  pressure  is  high  or  low,  care  should  be  exer- 
cised to  get  all  the  brushes  on  one  machine  at  a  uniform  pres- 
sure. If  the  pressure  is  unequal,  the  brushes  with  the  highest 
pressure  will  carry  the  highest  current.  In  many  cases  some 
brushes  on  a  machine  may  carry  three  or  four  times  as  much 
current  as  other  brushes,  due  to  this  variation  in  pressure. 
This  excessive  current  may  be  enough  to  burn  off  the  pigtails, 
overheat  the  brushes,  and  cause  glowing  and  pitting  of  the 
brush  faces.  The  best  value  of  brush  pressure  should  be  deter- 
mined for  any  machine  by  trial,  as  it  is  influenced  greatly  by 
local  conditions.  A  pressure  of  from  1.75  to  3  pounds  per 
square  inch  of  contact  surface  may  be  used  for  motors  and  gen- 
erators, and  from  four  to  seven  pounds  per  square  inch  for  such 
machines  as  crane  motors  and  railway  motors  where  vibration 
is  comparatively  severe.  However,  these  values  are  given  only 
as  an  indication  of  best  average  practice,  and  are  not  always 
the  best  to  use.  See  table  on  page  342,  paragraph  6  of  head- 
ing No.  I. 

Incorrect  Spacing  of  Brushes. — By  spacing  of  brushes,  is 
meant  the  distance  between  brushes  on  adjacent  studs  meas- 
ured around  the  commutator.  Thus,  in  a  four-pole  machine 
the  brushes  on  each  stud  should  be  at  a  distance  exactly  one- 
fourth  the  circumference  from  the  brushes  on  the  adjacent 
studs.  This  may  be  checked  up  by  counting  the  total  number 
of  commutator  bars,  then  dividing  by  the  number  of  poles,  and 
setting  the  brushes  that  number  of  bars  apart.  A  better  way, 
however,  is  to  obtain  a  piece  of  wrapping  paper  of  the  same 
length  as  the  circumference  of  the  commutator  and  of  the 
same  width  as  the  commutator.  Measure  off  the  correct 
brush  spacings  and  draw  lines  across  the  width  of  the  paper. 
Replace  this  on  the  commutator,  and  with  one  set  of  brushes 
set  so  as  to  toe  one  mark,  set  the  brushes  on  all  the  other  studs 
exactly  on  their  respective  marks.  If,  then,  the  sparking 
can  not  be  entirely  eliminated  by  shifting  the  brush  yoke  either 
forward  or  backward,  examine  the  field  coils  and  air  gaps. 


ADJUSTING  BRUSHES  AND  CORRECTING  TROUBLES       339 

Defective  Fields. — The  field  coils  on  the  machine  are  usually 
connected  in  series  so  that  a  part  of  one  coil  may  become  short- 
circuited  without  becoming  apparent  except  by  test.  Obtain 
a  voltmeter  with  a  suitable  scale  and  test  the  voltage  drop 
across  each  coil.  The  drop  should  be  the  same  for  each  field. 
If  one  coil  has  higher  drop  than  the  others,  the  coil  has  been 
incorrectly  wound.  If  one  coil  is  lower  than  the  other,  the 
coil  is  either  incorrectly  wound  or  a  portion  of  it  has  become 
short-circuited.  Either  case  will  require  a  new  coil. 

Another  more  common  defect  in  compound  machines  is  a 
reversal  of  one  of  the  series  field  poles.  For  this  test  use  a 
compass  and  check  the  shunt  as  well  as  the  series  coil.  First 
pass  current  through  the  shunt  coils.  Next,  bring  the  compass 
near  one  coil;  mark  it  either  South  or  North,  depending  on 
which  end  of  the  needle  is  attracted  to  the  field  pole.  If  the 
bearings  of  the  compass  are  not  perfectly  free,  use  care  not 
to  bring  the  needle  suddenly  too  close  to  the  pole,  as  it  is 
easy  to  reverse  the  magnetism  in  the  needles  of  moderate- 
priced  compasses.  After  marking  one  pole,  proceed  around  the 
machine  and  test  in  the  same  way  each  pole.  The  needle 
should  reverse  direction  at  each  pole.  If  two  adjacent  poles 
are  found  to  attract  the  same  end  of  the  needle,  one  coil  is 
reversed. 

Field  coils  can  also  be  tested  for  polarity  by  two  ordinary 
iron  nails  as  described  on  page  136. 

After  all  the  shunt  fields  have*  been  tested,  current  from 
some  source  should  be  passed  through  the  series  fields  only, 
care  being  taken  to  see  that  this  current  flows  in  the  same 
direction  as  when  the  machine  is  in  operation.  Nearly  all 
compound  machines  are  cumulative  compound,  in  which  case 
each  series  field  should  attract  the  same  end  of  the  needle  as  the 
shunt  field  attracts.  The  differential  compound  winding  is 
used  only  when  it  is  desired  to  have  a  rise  of  speed  with  an 
increase  of  load,  and  is  not  in  extensive  use.  In  such  machines, 
however,  the  series  fields  produce  poles  of  opposite  polarity 
to  those  produced  by  the  shunt  fields. 

Unequal  Air  Gaps. — Unequal  air  gaps  may  be  the  cause 
of  serious  trouble,  particularly  in  "lap"  or  "parallel  wound" 
machines.  Poor  centering  of  the  armature  when  the  machine 


340         ARMATURE  WINDING  AND  MOTOR  REPAIR 

was  assembled,  may  be  the  cause  or  it  may  result  later  from 
worn  bearings.  The  wear  may  be  either  at  the  bottom  of 
the  bearings,  due  to  the  weight  of  the  armature,  or  on  a  belt- 
connected  machine,  at  the  side  of  the  machine,  due  to  the 
pull  of  the  belt.  This  may  be  checked  mechanically  by 
measurement  with  thin  sheets  of  metal  or  fiber.  For  lap 
wound  machines  a  more  accurate  method,  however,  is  to 
disconnect  the  bus  bars  or  leads  which  connect  the  studs  of 
the  same  polarity.  Then,  without  any  load  on  the  machine, 
each  stud  will  be  independent,  and  with  a  low-reading  volt- 
meter any  difference  between  the  voltage  generated  under 
the  various  poles  can  be  detected.  This  is  valuable  on  a  lap 
wound  machine,  but  on  a  wave  wound  machine  the  variations 
are  equalized  and  the  mechanical  method  is  more  reliable. 

After  having  correctly  spaced  the  brushes,  checked  the 
drop  across  the  field  coils,  the  polarity  of  the  field  poles  and 
the  uniformity  of  the  air  gaps,  it  should  be  possible  to  shift 
the  brushes  to  a  point  where  no  sparking  will  appear  with 
a  steady  load  on  the  machine.  If  the  sparking  reappears 
with  a  variation  in  load,  either  the  brushes  are  of  incorrect 
thickness  or  the  neutral  field — that  is,  the  distance  between 
adjacent  pole  tips — is  too  small. 

Incorrect  Thickness  of  Brushes. — The  best  way  to  investigate 
this  sparking  is  with  a  voltmeter  reading  about  5  volts. 
Take  two  ordinary  lead  pencils  and  trim  down  one  side  of 
each  pencil  as  much  as  passible  without  exposing  the  lead. 
Near  the  top  of  each  pencil  cut  a  groove  deep  enough  to 
expose  the  lead  and  attach  a  piece  of  lamp  cord  or  other 
flexible  wire  to  each  pencil.  Remove  one  brush  and  hold 
the  pencils  in  the  brushholder  with  the  points  on  the  com- 
mutator and  the  flat  side  of  one  pencil  against  the  front  edge 
of  the  brushholder  and  the  flat  side  of  the  other  pencil 
against  the  back  edge  of  the  holder.  This  will  give  the  volt- 
age generated  across  the  brush,  or,  in  other  words,  the 
"commutation  voltage/'  Now  shift  the  brushes  to  a  point 
where  they  do  not  spark  and  read  the  "  commutation  voltage." 
Increase  the  load  until  the  brushes  spark  and  again  read  the 
" commutation  voltage."  If  it  has  decreased,  the  brushes  are 
too  narrow — that  is,  the  period  of  commutation  is  too  brief. 


ADJUSTING  BRUSHES  AND  CORRECTING  TROUBLES       341 

The  remedy  is  to  use  thicker  brushes  or  to  shift  the  brushes  with 
each  change  of  load.  This  condition,  however,  seldom  obtains. 

An  increase  in  the  commutation  voltage  indicates  that  the 
brushes  are  too  thick — that  is,  they  span  too  many  bars  or  the 
neutral  field  is  too  small.  This  thickness  of  the  brushes  may 
easily  be  changed  by  cutting  down  the  face  of  the  brush  with  a 
hack  saw  or  file,  leaving  the  body  of  the  brush  the  proper  size 
to  fit  the  brushholders.  This  will  help  the  trouble,  and  if  the 
sparking  does  not  entirely  cease,  it  may  be  necessary  to  widen 
the  neutral  field  by  increasing  the  distance  between  adjacent 
pole  pieces.  This  can  be  done  by  filing  the  edges  of  the  pole 
faces. 

Brushes  of  Wrong  Characteristics. — If  none  of  above  remedies 
cure  the  sparking,  the  machine  is  being  operated  with  brushes 
not  adapted  to  the  service.  The  next  step  should  be  to  give 
a  manufacturer  full  details  regarding  the  machine  and  ask 
for  recommendation  of  a  brush  best  adapted  to  the  machine 
and  service,  since  carbon  brush  manufacturers  maintain 
an  engineering  department  capable  to  give  valuable  advice 
along  this  line. 

As  a  guide  in  the  location  of  other  brush  troubles,  the  ac- 
companying table  will  be  found  useful. 

Possible  Causes  of  Brush  Troubles  on  Motors  and 

Generators  and  Their  Remedies  Compiled  by 

E.  H.  Martindale  of  the  National 

Carbon  Company 

I.  CAUSES  AND  REMEDIES  FOR  SPARKING  AT  BRUSHES 

1.  Brushes  off  electrical  neutral.     Shift  to  neutral  by  trial,  or.  set  on 
neutral  by  means  of  voltmeter. 

2.  Brushes  spanning  too  many  bars.     Trim  down  faces  of  brushes  for 
short  distance  back  from  end  or,  if  holders  are  clamp  type,  order  thinner 
brushes. 

3.  Brush  studs  not  parallel  with  the  commutator  bars.     Bend  the  brush 
studs  or  grind  or  shim  under  the  bolts  which  fasten  the  studs  to  the  yoke. 

4.  Incorrect  brush  spacing.     Check  the  spacing  by  counting  the  num- 
ber of  bars  between  studs  or  by  placing  a  strip  of  paper  around  the  com- 
mutator with  divisions  marked  off  equal  to  the  number  of  studs,  and 
correct  the  spacing  by  rotating  the  brush  studs  or  the  brushholders  on 
the  studs. 


342          ARMATURE  WINDING  AND  MOTOR  REPAIR 

5.  Brushes  tight  in  brush  holders.     Clean  the  holders  with  gasoline  and 
if  brushes  are  still  tight,  sandpaper  them  down  or  file  out  the  holders 
carefully. 

6.  Brush  pressure  too  low.     Pressure  should  be  1%  to  2%  Ib.  per  square 
inch  cross-section  for  stationary  motors  and  generators,  23^  to  4  Ib.  for 
elevator  and  mill  motors,  3  to  5  Ib.  for  crane  motors,  4  to  7  Ib.  forrail  way 
motors. 

7.  Too  low  contact  drop  of  brush.     Consult  a  brush  manufacturer. 

8.  Insufficient  abrasive  action  of  brushes.    Use  a  commutator  stone  or 
more  abrasive  brushes. 

9.  High  mica.     Use  abrasive  brushes,  a  commutator  stone  or  undercut 
the  mica. 

10.  Chattering.     See  heading  No.  VII  for  remedies. 

11.  Poor  adjustment  of  interpoles.    Consult  the  manufacturer  of  the 
machine. 

12.  Overloads.     Undercut  the  mica,  use  low-friction  brushes  and  check 
up  all  causes  for  short-circuit  currents  (see  heading  No.  IV)  to  reduce 
temperatures  as  much  as  possible. 

13.  Open  circuit  in  armature  coil.     Rewind  that  part  of  the  armature. 

14.  Loose  end  connection.     Scrape  and  resolder  all  defective  connections . 

15.  Worn  bearings.     Shim  or  renew  the  bearings. 

16.  Unequal  air  gaps.     Shim  the  short  poles  or  grind  off  the  faces  of 
the  long  poles,  or  if  from  worn  bearings,  see  paragraph  above. 

17.  Short-circuit  currents   be'ween   brush  studs  caused  by  unbalanced 
armature  winding.     Consult  the  manufacturer  of  the  irachine. 

18.  Eccentric  commutator  on  high-speed  machine.     Turn  or  grind. 

19.  Poor  belt  lacing.     Re-lace  or  still  better  use  a  continuous  belt. 

20.  Pound  of  reciprocating  engine  driving  the  machine. 

21.  Unstable  foundation. 

22.  Cross   currents  between  generators  operated  in  parallel  driven  by 
reciprocating  engines  due  to  variation  in  angular  speed  of  engines.     Use 
heavier  flywheel. 

II.  CAUSES  AND  REMEDIES  OF  FLAT  SPOTS  ON 
COMMUTATOR 

1.  Any  form  of  sparking.    See  heading  No.  1. 

2.  High  bar.    Tighten  the  commutator  bolts  and  turn  or  grind  the 
commutator. 

3.  Low  bar.     Use  commutator  stone  or  turn  or  grind  the  commutator. 

4.  Eccentric  commutator  on  high-speed  machine  causing  the  brush  to 
jump  from  the  commutator  at  the  high  spots.    Turn  or  grind  the  com- 
mutator. 

5.  Surges  of  load  current  due  to  short-circuit  on  the  line  or  an  instanta- 
neous high  peak  load. 

6.  Mechanically  unbalanced  armature.     Place  on  balancing  ways  and 
add  weight  at  lightest  point. 


ADJUSTING  BRUSHES  AND  CORRECTING  TROUBLES       343 

7.  Difference  in  hardness  of  commutator  bars.     Undercut  the  mica  and 
use  non-abrasive  brushes. 

8.  Difference  in  hardness  of  mica.     Undercut  the  mica  and  use  non- 
abrasive  brushes. 

III.  CAUSES  AND  REMEDIES  FOR  BLACKENING  OF 
COMMUTATOR 

1.  Sparking.     See  heading  No.  I  for  causes  and  remedies. 

2.  Too  much  lubricant.     Clean  commutator  with  gasoline. 

IV.  CAUSES  OF  HEATING  IN  A  MOTOR  OR  GENERATOR 

WITH  REMEDIES 

1.  Severe  sparking.     See  heading  No.  I. 

2.  Short-circuit  currents. 
(a)  Brushes  off  neutral. 
(6)  Faulty  brush  spacing. 

(c)  Too  thick  brushes. 

(d)  Unequal  air  gaps. 

(e)  Crooked  brush  studs. 

(/)  Too  low  contract  drop  of  brushes. 
(g)  Unbalanced  armature. 
For  remedies  of  items  a  to  g  see  same  causes  under  heading  No.  1. 

3.  Too  high  or  too  low  brush  pressure.     See  paragraph  6  under  heading 
No.  I. 

4.  High-friction  brushes.     Undercut  mica  and  use  low  friction  brush. 

5.  Commutator  too  small.     Consult  manufacturer  of  machine. 

6.  Too  high  a  ratio  of  brush  area  to  commutator  surface.    Use  fewer 
brushes  of  higher  carrying  capacity  and  lower  friction. 

7    Overloads.    See  paragraph  12  under  heading  No.  I. 
8.  Chattering  of  brushes.     See  heading  No.  VII. 

V.  CAUSES  AND  REMEDIES  FOR  HONEY-COMBING  OF 

BRUSH  FACES 

1.  Short-circuit  currents.     See  paragraph  2  under  heading  No.  IV. 

2.  Too  low  brush  pressure.     See  paragraph  6  under  heading  No.  I. 

3.  Brushes  of  insufficient  carrying  capacity.    Consult  a  brush  manu- 
facturer. 

VI.  CAUSES  AND  REMEDIES  FOR  BRUSHES  PICKING 
UP  COPPER 

1.  Heavy  short-circuit  currents.     See  paragraph  2  under  heading  No.  IV. 

2.  Sand  under  the  brush  faces.     Wipe  brush  face  carefully  after  sand- 
papermg  either  brushes  or  commutator. 

3    Commutator  not  thoroughly  cleaned  after  turning.     Finish  the  surface 
with  a  commutator  stone  after  turning. 


344         ARMATURE  WINDING  AND  MOTOR  REPAIR 

4.  Collection  of  copper  dust  by  lubricant  in  abrasive  brushes.     Undercut 
the  mica  and  use  non-abrasive  brushes. 

5.  Electrolytic  action.    Change  the  grade  of  brush ;  better  consult  a 
brush  manufacturer. 

VII.  CAUSES  AND  REMEDIES  FOR  BRUSHES  CHATTERING 

1.  High-friction  brushes.     Change  the  grade  or  pressure. 

2.  Rough  commutator.    Use  a  commutator  stone. 

3.  Dirty  commutator.    Clean  with  gasoline. 

4.  High  mica.     Use  a  commutator  stone  or  undercut  the  mica. 

5.  Wide   slots   with   thin   brushes.     Fill   the   slots   with    commutator 
cement. 

6.  High  bars.     Tighten  the  commutator  bolts  and  turn  or  grind  the 
commutator. 

7.  Flat  spots.    Use  commutator  stone  unless  the  flat  spots  are  too 
large  for  stoning,  in  which  case  turn  or  grind  the  commutator. 

8.  Brush  operating  at  the  wrong  bevel,  frequently  found  where  brushes  are 
operating  in  a  stubbing  position  with  angles  of  less  than  20  degrees.     Change 
the  grade  of  brush  or  angle  of  operation.     Better  consult  a  brush  manu- 
facturer or  the  manufacturer  of  the  machine. 

VIIL  CAUSES  AND  REMEDIES  FOR  LOOSENING  OF 
BRUSH  SHUNTS 

1.  Poor  workmanship  in  attaching  shunts. 

2.  Insufficient  carrying  capacity.    Consult  a  brush  manufacturer. 

3.  Heating.     See  heading  No.  IV. 

4.  Vibration.     See  heading  No.  VII. 

5.  Combination  of  heating  and  vibration. 

6.  Loose  terminal  screws  causing  unequal  distribution  of  load. 

7.  Unequal  brush  pressure  causing  unequal  distribution  of  load.     See 
that  all  brush  pressures  are  uniform  and  conform  to  recommendations 
given  in  paragraph  6  under  heading  No.  I. 

8.  Heavy  short-circuit  currents  between  different  brushes.     See  b,  d,  e,  g, 
of  paragraph  2,  under  heading  No.  IV. 


CHAPTER  XIV 

INSPECTION  AND  REPAIR  OF  MOTOR  STARTERS, 
MOTORS  AND  GENERATORS 

On  account  of  the  variety  of  troubles  that  may  be  corrected 
in  a  simple  way  at  one  time  but  require  more  extensive  repairs 
at  others,  it  is  a  very  difficult  matter  to  lay  down  hard  and  fast 
rules  that  will  always  work  out  on  every  repair  job  when  fol- 
lowed to  the  letter.  The  ability  to  know  when  temporary 
repairs  will  suffice  and  when  a  permanent  job  must  be  done 
at  once,  comes  largely  from  experience.  The  term  repairman 
has  been  used  throughout  this  book  as  a  title  that  a  good  engi- 
neer can  bear  with  pride  when  he  measures  up  to  all  the  quali- 
fications of  the  man  who  in  the  majority  of  cases  knows  what 
to  do  and  just  how  to  do  it  and  seldom  guesses  without  a  good 
percentage  of  the  probabilities  of  being  right  in  his  favor. 
The  main  difference  between  the  designer  and  the  repairman 
is  that  the  former  must  know  what  to  do  while  the  latter  must 
know  -how  to  do  it.  A  capable  repairman  combines  both 
qualifications  through  years  of  experience.  One  of  the  best 
ways  of  saving  time  for  the  young  engineer  entering  the  repair 
field,  is  to  serve  as  an  apprentice  with  a  large  electrical  manu- 
facturing company.  In  this  way  through  association  with 
those  who  design  and  those  who  build  and  test,  a  great  deal 
of  information  is  absorbed  that  only  years  of  experience  from 
one  job  at  a  time  will  make  possible.  This  fund  of  information 
is  essential  in  an  intelligent  discussion  of  any  repair  job  or  the 
presentation  of  rules  or  suggestions  for  looking  for  the  trouble 
and  then  actually  making  the  repair. 

In  the  accompanying  pages  there  are  presented  suggestions 
for  the  repairman  in  the  inspection  and  repair  of  motors  and 
generators  prepared  by  H.  S.  Rich,  at  the  suggestion  of  the 
author  and  published  in  the  Electrical  Record,  October,  1918 
to  April,  1919.  These  suggestions  are  accompanied  with  ex- 
ploded views  of  the  device  discussed,  so  that  the  repairman  can 

345 


346         ARMATURE  WINDING  AND  MOTOR  REPAIR 

form  a  mental  picture  of  the  work  described  which  will  approxi- 
mate as  closely  as  possible  the  impression  that  would  be  se- 
cured by  actually  doing  the  work. 

Cost  of  Repairs  for  Polyphase  Motors. — For  the  purpose 
of  furnishing  estimates  for  repairing  polyphase  motors  that 
will  guide  the  repairman  who  has  had  a  limited  experience  of 
repairwork,  George  A.  Schneider  has  compiled  the  accompany- 
ing table  (Journal  of  Electricity,  May  1,  1917)  from  costs  of 
actual  repair  jobs  which  he  has  handled.  The  table  as  pre- 
sented has  been  revised  to  take  into  consideration  the  cost 
of  materials  entering  into  repairs  for  the  year  1917.  The 
data  refers  particularly  to  60-cycle,  two  and  three-phase, 
squirrel-cage  motors  wound  for  the  standard  voltages  of  110 
to  550. 

For  most  of  the  sizes  listed  the  costs  were  arrived  at  by 
taking  the  average  cost  of  repairs  for  a  given  frame  and  then 
applying  this  cost  to  the  various  ratings  built  in  that  frame. 
This  will  be  apparent  by  comparing  the  costs  for  the  different 
ratings.  Take  for  example,  frame  G.  The  cost  of  rewinding 
the  stator  is  $34.75.  This  figure  has  been  applied  to  the 
following  ratings  all  of  which  are  built  in  that  frame :  1  horse- 
power, 900  revolutions  per  minute;  1.5  horsepower,  1200  revo- 
lutions per  minute,  and  3  horsepower,  1800  revolutions  per 
minute.  The  frame  sizes  specified  do  not  apply  to  any  par- 
ticular line  of  motors,  but  were  arbitrarily  chosen  for  the 
purpose  of  this  table.  However,  the  relative  output  of  a 
given  frame  at  the  different  speeds  will  be  found  to  agree  quite 
closely  with  several  lines  of  induction  motors  on  the  market. 

These  estimates  may  also  be  used  equally  well  for  motors  of 
other  frequencies  by  taking  the  figures  applying  to  a  60-cycle 
rating  built  in  the  same  frame.  This  comparison  can  be 
easily  made  by  referring  to  the  manufacturer's  rating  and 
dimension  sheets  for  thaf  particular  line  of  motors.  The 
tables  may  be  further  applied  to  slip-ring  or  phase-wound 
motors,  since  the  cost  of  rewinding  the  rotor  of  such  a  machine 
will  not  differ  materially  from  the  cost  of  rewinding  its  stator. 
On  this  basis  the  cost  of  completely  rewinding  a  10  horsepower, 
1800  revolutions  per  minute  slip-ring  motor  built  in  frame  / 
will  be  $119,  or  $59.50  for  the  rotor  or  stator  separately. 


INSPECTION  OF  MOTORS  AND  GENERATORS         347 


COST  OF  REPAIRS  FOR  GO-CYCLE  POLYPHASE  MOTORS  BASED  ON  A  LARGE 
NUMBER  OF  REPAIR  JOBS 


Horse- 
power 

Syn- 
chronous 
speed  in 
rpm. 

Frame 
size 

Rewinding 
stator 

Re- 
soldering 
rotor 

Bearing 
linings, 
per  set  of 
two 

Paint- 
ing 

Re- 
crating 

0.50 

1200 

C 

$26.25 

$2.50 

$1.35 

$1.00 

$1.00 

0.50 

1800 

A 

24.25 

2.25 

.35 

1.00 

.00 

0.75 

1200 

E 

28.00 

3.00 

.85 

.00 

.00 

0.75 

1800    , 

B 

24.25 

2.25 

.35 

.00 

.00 

1.00 

900  ' 

G 

34.75 

4.00 

.10 

.50 

.50 

1.00 

1200 

F 

28.50 

3.00 

.85 

.25 

.00 

1.00 

1800 

C 

26.25 

2.50 

.35 

.00 

.00 

1.50 

1200 

G 

34.75 

4.00 

3.10 

.50 

.50 

1.50 

1800 

E 

28.00 

3.00 

1.85 

.00 

.00 

2.00 

1200 

G 

34.75 

4.00 

3.10 

.50 

.50 

2.00 

1800 

F 

28.50 

3.00 

1.85 

.25 

.00 

3.00 

900 

I 

53.50 

6.50 

5.25 

.50 

.50 

3.00 

1200 

H 

48.50 

6.75 

3.55 

.50 

.50 

3.00 

1800 

G 

34.75 

4.00 

3.10 

.50 

.50 

5.00 

900 

K 

73.75 

8.75 

8.05 

.75 

2.00 

5.00 

1200 

I 

53.50 

6.50 

5.25 

.50 

1.50 

5.00 

1800 

H 

48.50 

4.75 

3.55 

.50 

1.50 

7.50 

900 

L 

70.75 

12.00 

7.85 

.00 

2.50 

7.50 

1200 

J 

59.50 

7.00 

6.60 

.75 

2.00 

7.50 

1800 

I 

53  .  50 

6.50 

5.25 

.50 

1.50 

10.00 

900 

M 

75.00 

13.25 

7.85 

2.00 

2.50 

10.00 

1200 

L 

70.75 

12.00 

7.85 

2.00 

2.50 

10.00 

480 

J 

59  .  50 

7.00 

6.60 

1.75 

2.00 

15.00 

720 

P 

93.75 

15.50 

10.25 

3.00 

4.00 

15.00 

900 

N 

71.25 

14.25 

10.25 

3.00 

4.00 

15.00 

1200 

M 

75.00 

13.25 

7.85 

2.00 

2.50 

15.00 

1800 

K 

73.75 

8.75 

8.05 

1.75 

2.00 

20.00 

600 

S 

156.25 

19.00 

12.10 

3.25 

6.00 

20.00 

900 

P 

93.75 

15.50 

10.25 

3.00 

4.00 

20.00 

1200 

N 

71.25 

14.25 

10.25 

3.00 

4.00 

20.00 

1800 

M 

75.00 

13.25 

7.85 

2.00 

2.50 

25.00 

600 

S 

156.25 

19.00 

12.10 

3.25 

6.00 

25.00 

720 

S 

156.25 

19.00 

12.10 

3.25 

6.00 

25.00 

900 

R 

143.75 

17.75 

12.00 

3.25 

6.00 

25.00 

1200 

P 

93.75 

15.50 

10.25 

3.00 

4.00 

35.00 

600 

T 

18.7  .  50 

20.50 

19.95 

3.50 

6.25 

35.00 

720 

S 

156.25 

19.  CO 

12.10 

3.25 

6.00 

35.00 

900 

S 

156.25 

19.00 

12.10 

3.25 

6.00 

35.00 

1200 

R 

143.75 

17.75 

12.00 

3.25 

6.00 

50.00 
50.00 

600 
720 

V 
V 

218.75 
218.75 

21.75 
21.75 

30.85 
30.85 

3.50 
3.50 

6.25 
6.25 

50.00 

900 

T 

187.50 

20.50 

19.95 

3.50 

6.25 

50.00 

1200 

S 

156.25 

19.00 

12.10 

3.25 

6.00 

Points  to  Consider  when  Estimating  Cost  of  a  Motor  Repair 
Job. — The  estimates  for  rewinding  the  stator  or  resoldering 
the  rotor  given  in  the  accompanying  table  do  not  include  any 


348          ARMATURE  WINDING  AND  MOTOR  REPAIR 

preliminary  work  required  to  put  the  stator  structure  in  fit 
condition  to  receive  the  new  winding  or  work  required  on  the 
rotor  before  the  actual  resoldering  can  be  started.  In  other 
words,  the  figures  cover  only  the  actual  rewinding  or  resolder- 
ing, as  the  case  may  be.  However,  this  preliminary  work  is 
frequently  necessary  and  must  always  be  considered  in  making 
up  estimates.  It  is  due  to  a  number  of  causes. 

For  example,  the  motor  bearing  linings  may  have  worn 
down  sufficiently  to  allow  the  rotor  to  rub  against  the  stator. 
If  the  motor  has  operated  very  long  in  this  condition  the  lami- 
nations of  either  or  both  stator  and  rotor  will  probably  be 
damaged,  which  may  require  considerable  work  to  put  them 
into  their  original  condition.  Again,  a  defective  or  broken 
bearing  may  injure  the  shaft.  Sometimes  this  damage  will  be 
serious  enough  to  require  a  new  shaft.  New  bearing  linings  will 
probably  be  required  in  either  case.  Burned-out  windings 
may  also  be  accompanied  by  fusing  of  parts  of  the  stator  lamina- 
tions. These  fused  portions  must  necessarily  be  removed 
before  actual  replacement  of  the  coils  can  be  commenced. 

In  a  rotor  which  has  been  badly  overheated,  allowing  the 
melted  solder  to  be  thrown  out,  arcing  is  frequently  set  up 
between  the  rotor  bars  and  the  end  rings  causing  serious 
burning.  When  this  occurs,  new  end  rings  are  often  needed 
either  for  one  or  both  ends  of  the  rotor  or  perhaps  part  of  the 
bars  will  have  to  be  replaced.  With  bolted  end  ring  construc- 
tion there  is  also  liability  of  trouble.  The  expansion  of  the 
end  rings  caused  by  the  excessive  heat,  tends  to  snap  the  bolts 
between  the  rotor  bars  and  the  rings,  producing  the  most 
favorable  conditions  for  arcing.  Burnouts  of  this  kind, 
for  either  soldered  or  bolted  construction,  are  quite  common 
in  connection  with  motors  which  have  been  started  from 
time  to  time  under  loads  which  have  required  heavy  starting 
torque  with  long  periods  of  acceleration.  Two-  or  three- 
phase  motors  allowed  to  operate  single-phase  for  a  considerable 
length  of  time  may  also  develop  troubles  of  this  kind.  Very 
often  the  rotor  will  be  badly  damaged  while  the  stator  has 
only  been  only  slightly  overheated.  Conversely,  in  some 
cases  the  stator  will  be  burned  out  while  the  rotor  is  uninjured. 

From  these  points  it  will  be  clear  that  estimates  for  repairing 


INSPECTION  OF  MOTORS  AND  GENERATORS         349 

motors  should  not  be  made  until  after  the  motor  has  been 
given  a  careful  inspection  for  otherwise  there  is  liable  to  be  a 
wide  discrepancy  between  the  estimated  and  the  actual  cost 
of  making  the  repair.  In  furnishing  an  estimate  under 
conditions  where  a  detailed  inspection  is  not  possible,  details 
of  what  the  estimate  covers  should  be  given  with  a  notation 
of  additional  repairs  that  may  be  necessary  after  an  inspection. 
The  data  of  the  table  on  page  347,  is  based  on  a  large  number 
of  repair  jobs  -and  is  conservative  for  labor  and  material 
conditions  in  1917. 


I.    INSPECTION   AND    OVERHAULING    OF   DIRECT -CURRENT 
MOTOR  STARTERS 

The  following  points  to  be  considered  by  the  repairman 
when  inspecting  and  repairing  a  motor  starter  are  given  by 
T.  H.  Reardon  (Electrical  Review,  May  25,  1918). 


I 

FIG.  240. — A  starting  rheostat  with  no  voltage  release  for  use  with  shunt  and 
compound  direct-current  motors. 

In  direct-current  motor  service  the  ordinary  rheostat 
in  which  an  arm  moves  over  contacts  arranged  on  the  arc  of  a 
circle  is  the  most  convenient  starting  device.  All  such  rheo- 
stats are  provided  with  an  electromagnet  which  holds  the 
arm  in  place  after  it  has  been  brought  up  to  the  last  notch 
or  full  running  position.  This  magnet  loses  its  power  if  the 


350          ARMATURE  WINDING  AND  MOTOR  REPAIR 

current  goes  off  the  line  and  the  arm  flies  back  to  the  starting 
position.  The  arm  is  returned  to  the  starting  position  by  a 
spring,  which  is  always  acting  in  opposition  to  the  pull  of 
the  magnet  that  tends  to  keep  the  arm  in  last-notch  position 
as  long  as  the  magnet  is  energized.  If  the  pull  of  the  spring 
is  too  great,  which  somtimes  occurs  when  an  inexperienced 
man  makes  certain  changes  in  the  way  of  correcting  things, 
the  pull  of  the  magnet  will  not  be  sufficient  to  hold  the  arm 
in  place  when  it  is  brought  up  and  the  arm  will  fly  back. 
Conversely,  if  the  i^rength  of  the  spring  becomes  reduced, 
which  often  happens  when  motors  are  placed  in  damp  places, 
or  worse  still,  where  dampness  and  certain  bleaching  agents 
(such  as  chlorine  and  oxides  of  chlorine)  act  jointly  in  bringing 
about  metallic  deterioration  in  apparatus,  steel  springs  will 
be  found  corroded  to  such  an  extent  that  they  do  not  possess 
their  original  strength  and  elasticity.  As*  a  preventive 
measure,  springs  as  well  as  all  other  metal  parts  exposed  to 
corrosive  action  should  receive  a  drop  or  two  of  a  mineral 
oil  or  be  slightly  smeared  with  vaseline  occasionally  to  prevent 
such  deterioration.  The  pull  of  the  spring  and  the  pull  of 
the  magnet  are  balanced  against  each  other,  but  there  are 
other  conditions  that  must  be  taken  into  account  in  making 
adjustments. 

After  the  current  ceases  to  flow  in  the  low-voltage  release- 
magnet  coils,  which  will  not  be  until  the  motor  comes  to 
rest  (the  motor  acting  as  a  generator  will  maintain  this  current 
until  the  motor  stops),  the  magnet  will  still  exert  a  considerable 
pull  on  its  armature  due  to  the  relatively  large  amount  of 
residual  magnetism  that  remains  in  the  core  of  an  electro- 
magnet after  the  current  has  ceased  to  circulate  and  before  the 
armature  is  pulled  away  from  the  magnet  poles.  An  arm 
that  will  not  fly  back  to  starting  point  when  the  motor  stops, 
if  moved  back  by  hand  and  then  brought  up  again,  will  not 
stick.  In  certain  cases,  however,  it  may  stick  and  when  it  does 
do  so  it  is  not  due  to  any  pull  exerted  by  the  magnet  but  it  will 
probably  be  found  that  the  sliding  contact  on  the  bottom  of  the 
arm  bears  so  hard  against  the  contacts  that  the  strength 
of  the  spring  is  not  sufficient  to  overcome  this  braking  effect 
due  to  the  friction  of  the  shoe  moving  over  the  contacts. 


INSPECTION  OF  MOTORS  AND  GENERATORS         351 

Sometimes  this  friction  effect  may  exist  only  on  one  contact, 
one  contact  standing  higher  than  the  rest,  and  if  this  particular 
contact  happens  to  be  next  to  the  magnet,  the  movement 
of  the  arm  will  be  retarded  before  the  arm  has  a  chance  to 
acquire  any  momentum  that  would  carry  it  back  provided 
it  once  got  started. 

If  the  magnet  pull  due  to  residual  magnetism  after  current 
circulation  ceases  should  be  responsible  for  the  arm  sticking 
after  the  motor  stops  (this  will  very  rarely  be  the  case,  how- 
ever), the  keeper  can  be  lightly  tinned  over  with  a  soldering 
iron,  thus  placing  a  certain  amount  of  reluctance  in  the  mag- 
netic circuit  of  the  magnet  and  its  keeper.  A  drop  of  oil 
occasionally  on  the  stud  on  which  the  arm  is  pivoted  will 
help  in  securing  free  movement. 


II.  INSPECTION  AND  OVERHAULING  OF  AUTO-STARTERS  FOR 
A.-C.  MOTORS 

When  examining  and  testing  an  auto-starter  to  locate 
troubles,  T.  H.  Readron  (Electrical  Review,  May  25,  1918) 
mentions  the  following  points  as  possible  causes  of  the  troubles : 

Auto-starters  are  generally  used  for  starting  alternating- 
current  motors  of  the  induction  type  when  the  motors  are 
above  five  horsepower.  Small  motors  are  usually  thrown  di- 
rectly on  the  line  without  any  starting  device  and,  although 
the  starting  current  is  four  or  five  times  greater  than  the 
normal  current,  the  fluctuation,  as  a  rule,  is  not  serious  enough 
to  necessitate  the  use  of  an  auto-starter.  Usual  practice  is  as 
follows:  Polyphase  induction  motors  up  to  5  hp.  are  thrown 
directly  on  the  line.  Motors  of  7.5  to  30  hp.  are  started 
by  means  of  a  star-delta  switch  which  is  not  an  induction 
starter  but  simply  a  switch  operating  in  oil  and  changing  the 
connections  of  the  motor  from  star  on  starting  to  delta  on 
running.  The  motor  in  that  case  is  provided  with  six  termi- 
nals to  accommodate  the  change.  Auto-starters  or  auto- 
transformers  are  mostly  used  for  squirrel-cage  induction 
motors  of  considerable  size  or  over  30  hp. 

The  auto-starter  being  adapted  for  alternating-current 
work,  differs  from  the  rheostat  in  that  it  possesses  not  only 


352 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


some  resistance  but  considerable  reactance  in  damping 
current  flow,  while  the  rheostat  possesses  resistance  only. 
Moreover,  the  rheostat  is  a  step-by-step  starting  device; 
its  arm  is  to  be  moved  slowly  over  the  contacts,  while  the 
auto-starter  has  but  one  step  from  starting  to  running  and 
the  handle  should  always  be  thrown  promptly  from  one  posi- 
tion to  the  other  position.  • 

The  accompanying  diagram  (Fig. 
242)  shows  the  plan  of  the  auto-starter 
as  usually  constructed.  Six  wires  are 
brought  to  the  rocker  cylinder  of  the 
switch,  which  is  moved  by  the  switch 
handle.  One  wire  is  a  feed  or  line 
wire,  the  wire  next  goes  to  the  motor, 
the  next  wire  is  line  and  the  next  one 
motor  again,  etc.  When  the  switch 
is  thrown  to  the  starting  position,  the 
six  contacts  on  the  switch  cylinder 
meet  six  contacts  on  the  starting 
block  and  the  current  from  a  line 
contact  on  the  switch  cylinder  passes 
to  the  contact  on  the  starting  block 
through  one  coil  of  the  reactance  and 
back  to  the  next  contact  on  the  switch 
cylinder,  and  thence  to  the  motor. 

When  the  handle  is  thrown  to  the 
running  position,  only  three  contacts 
on  the  switch  cylinder  make  contact — 

these  are  the  motor  contacts  on  the  switch  cylinder  and  they 
meet  three  contacts  on  the  running  block,  which  contacts  are 
directly  connected  to  the  line  wires  either  through  fuses  or 
overload  relays. 

The  line  contacts  on  the  switch  cylinder,  when  the  cylinder 
is  thrown  to  running  position,  do  not  connect  with  anything — 
they  stand  clear  or  dead-ended.  These  contacts  on  the  switch 
cylinder  can  be  identified  by  using  a  lamp  bank  and  making 
contact  with  them,  the  switch  handle  being  in  off  position. 
A  light  will  be  obtained  between  line  and  line. 

The  first  trouble  to  look  for  is  burned  or  imperfect  contacts. 


FIG.  241.  —  An  auto- 
starter  or  compensator  for 
use  with  alternating-current 
squirrel  cage  induction 
motors. 


INSPECTION  OF  MOTORS  AND  GENERATORS         353 

After  taking  the  oil  pan  off  the  switch  contacts  can  be  inspected 
without  any  trouble. 

If  the  contacts  are  burned  or  rough,  they  should  be  taken 
off  and  filed  smooth  or  replaced  with  new  ones.  The  same 
applies  to  the  fingers  that  meet  the  contacts.  The  handle 
should  be  thrown  to  one  position  and  then  the  fingers  should 
be  tried  to  see  that  they  press  firmly  and  evenly  against  the 
contacts  on  the  switch  cylinder.  If  they  do  not,  throw  the 
switch  handle  to  off  position  and  they  can  easily  be  bent  in- 
ward sufficiently  to  make  a  firm  contact.  It  will  be  well  not 
to  bend  them  too  much  at  once,  for  if  this  is  done,  they  will 
not  slide  over  the  switch  cylinder  contacts  properly. 


I |l^Finish 

Side  View  ^Start  Y- Connected 

FIG.  242. — Connections  for  a  common  form  of  auto-starter  for  alternating- 
current  motors. 

In  regard  to  broken  wires,  the  wires  that  are  attached  to  the 
switch  cylinder,  six  in  number,  are  bent  slightly  every  time 
that  the  switch  handle  is  moved.  These  wires  now  and  then 
break  off.  The  other  six  wires  that  enter  the  auto-starter 
go  to  fixed  immovable  contacts  and  rarely  cause  trouble. 
To  save  labor,  test  everything  out  as  far  as  possible  with  the 
lamp  bank  or  some  other  way  equally  good,  depending  upon 
what  kind  of  testing  apparatus  there  is  at  hand. 

It  will  be  well  to  bear  in  mind  that  a  ring  obtained  with  a 
magneto  or  a  light  obtained  with  a  lamp  bank  is  not  always 
conclusive.  Instances  are  quite  common  where  a  wire  01 
cable  breaks  inside  of  the  insulation  and  there  afterward  re- 
mains a  sheath  of  metallic  oxide  or  smudge  that  will  pass  cur- 

23 


354         ARMATURE  WINDING  AND  MOTOR  REPAIR 

rent  enough  to  give  a  dull  light  on  a  lamp  bank  or  a  feeble 
ring  on  the  magneto.  When  it  is  decided  that  a  certain  wire 
is  broken,  take  out  the  two  screws  that  hold  that  particular 
contact  to  the  switch  cylinder  and  pull  down  the  terminal 
with  a  pair  of  pliers. 

If  the  wire  is  intact,  it  will  resist  a  strong  pull.  If  the  wire 
is  broken  and  is  held  by  the  insulation,  it  will  easily  pull 
apart.  If  it  pulls  down  through  the  switch  cylinder  suffi- 
ciently, attach  a  piece  of  new  wire  or  cable  to  it  and  use  it 
for  a  snake  to  draw  the  new  wire  into  place,  pulling  at  the  top 
where  the  wires  enter  the  auto-starter.  If  this  does  not  work, 
draw  the  broken  wire  out  at  the  top  at  all  events  and  get  a 
piece  of  stiff  brass  or  steel  wire,  about  No.  14  gauge.  Bend  a 
smooth  small  loop  on  the  end  of  it  and  pass  it  down  from 
above.  Have  a  helper  with  a  small  hook  wire  watch  for  it  and 
hook  it  above  the  switch  cylinder  so  that  it  can  be  passed 
through  the  proper  hole  in  the  switch  cylinder.  When  this  is 
done,  the  terminal  can  be  soldered  on  at  the  lower  wire  and  the 
terminal  drawn  back  and  wire  spliced  at  the  stop  of  the  auto- 
starter.  It  should  always  be  the  aim  to  make  such  a  repair 
if  possible  without  disassembling  the  parts  of  the  auto-starter 
as  such  parts  go  back  with  difficulty. 

It  may  be  advisable  or  even  necessary  to  use  a  wire  slightly 
smaller  than  the  original  one  in  order  to  get  through  tight 
places.  There  will  be  no  decided  objection  to  doing  this  as  the 
cross-section  of  copper  is  always  sufficiently  ample  to  justify 
a  slight  reduction  when  such  a  reduction  in  size  is  really 
necessary. 

The  following  method  of  insulating  leads  on  auto-starters 
will  prevent  siphoning  of  oil: 

(a)  Remove  insulation  from  each  lead  just  above  the  highest  oil  level 
for  a  distance  of  two  inches. 

(6)  Sweat  the  strands  of  the  cable  thoroughly  together  so  as  to  close 
up  all  spaces  between  the  conductors  for  a  distance  of  one  inch. 

(c)  Insulate  the  lead  with  treated  cloth  tape,  wrapping  the  tape  tightly 
around  the  conductor  and  brushing  each  layer  with  insulating  varnish 
while  wrapping. 

(d)  Extend  the  wrapping  to  tape  with  three  overlapping  layers  at 
least  one  inch  on  the  insulation  at  each  end  of  the  bare  section. 


INSPECTION  OF  MOTORS  AND  GENERATORS 


355 


III.   INSPECTION  AND  OVERHAULING  OF  DRUM  TYPE 
CONTROLLERS 

The  drum  type  controller  is  used  with  variable  speed  shop 
motors,  on  trolley  cars,  elevated,  subway  and  railway  trains, 
on  cranes,  and  on  some  types  of  elevators.  This  controller 
may  be  mounted  in  any  position  so  as  to  permit  handy  con- 
trol. On  trolley  and  railway  cars,  it  is  placed  vertically  with 
the  lever  on  top.  For  shop  use  it  is  sometimes  mounted  on  a 
machine  or  on  the  wall  upside  down,  being  well  out  of  the  way 
and  yet  having  its  lever  within  reach.  On  some  elevators  it  is 


2  — 
RESISTANCE.  6  — 


CONTROLLER. 

FIG.  243. — Gridiron  resistance  used  with  drum  type  controller  shown  at 
the  right  for  variable  speed  motors.  (Figures  refer  to  numbers  of  para- 
graphs of  text.) 

installed  horizontally  and  operated  with  a  steel  cable  running 
over  a  grooved  sheave  wheel.  Some  drum  type  controllers 
have  only  a  single  arrangement  of  segments  with  the  reversing 
connections  operated  by  a  separate  lever,  while  others  have 
duplicate  segments,  half  of  which  are  arranged  to  operate  on 
reverse. 

1*.  The  hard  usage  to  which  a  controller  is  subjected  usually 
will  loosen  some  of  its  screws  or  connections,  so  these  should 
be  looked  over  often  and  kept  tight.  The  line  connections 

*  The  following  paragraph  numbers  refer  to  the  parts  in  Figs.  243  and 
244. 


356          ARMATURE  WINDING  AND  MOTOR  REPAIR 

to  the  fingers  and  the  segments  themselves  require  the  most 
attention.  A  finger  generally  carries  a  copper  block  of  various 
bevelled  shapes,  riveted  to  a  stiff  phosphor  bronze  strip  which 
provides  the  necessary  tension  and  is  regulated  by  an  adjusting 
screw.  The  fingers  should  be  provided  with  flexible  stranded 
copper  pig-tails  which  carry  the  current.  If  the  spring  carries 
the  current,  it  is  liable  to  heat  and  loose  its  tension.  Set  the 
fingers  so  that  they  can  not  be  jammed  under  a  segment  instead 
of  riding  upon  it.  If  any  are  badly  worn  renew  them,  like- 
wise overheated  springs  and  defective  pig-tails. 

2.  The  primary  segments  for  induction  motor  control  and 
the  field  segments  for  direct-current  motors  do  not  carry  as 
much  current  as  the  armature  or  slip  ring  segments  and  are 
thus  subject  to  less  wear.     The  ones  mostly  worn  are  in  series 
with  the  slip  rings  or  the  direct-current  brushes,  thus  if  con- 
tacts are  very  poor  the  motor  will  not  operate  or  it  may  run 
slowly  if  on  alternating  current.     If  the  copper  segments  are 
badly  worn,  replace  them  with  new  ones  having  the  proper 
curvature  and  with  counter  sunk  screws  tightly  fastened  down. 
A  slight  lubrication  of  vaseline  is  good  for  segments. 

3.  The  direct-current  controller  should  supply  the  motor 
shunt  field  with  current  on  the  first  point,  likewise  the  alter- 
nating-current controller  should  close  the  circuit  to  the  induc- 
tion motor  primary  winding  either  before  or  at  the  same  instant 
that  the  armature  receives  current.     If  these  contacts  are  not 
properly  made  at  the  first  point,  fuses  are  liable  to  blow  as  the 
result  of  no  field. 

4.  Insulating  partitions  are  often  provided  between  the 
segments  to  prevent  arcing  over.     If  these  are  badly  burned, 
renew  them. 

5.  Controller    diagram    connections    which    are    generally 
pasted  inside  of  the  cover  are  worth  saving.     If  constant  arcing 
is  liable  to  deface  the  diagram,  either  provide  a  duplicate  for 
reserve  or  remove  the  one  from  the  cover  and  mark  on  it  the 
controller  number,  etc.,  for  identification. 

6.  Test  with  a  magneto  on  a  dead  controller,  or  with  a  test 
lamp  on  a  live  one  for  any  possible  stray  ground  to  the  casing. 
It  should  test  free,  as  a  ground  would  likely  bother  the  operator. 

7.  The  controller  resistance  is  usually  of  the  gridiron  type 


INSPECTION  OF  MOTORS  AND  GENERATORS 


357 


which  allows  any  speed  continuously.  This  resistance  has  no 
connection  with  the  primary  winding  or  line  segments.  See 
that  no  grids  are  broken,  as  they  are  quite  frail.  A  few  extras 
in  stock  are  handy.  Test  the  resistance  for  a  ground  to  its 
own  frame.  A  perforated  sheet  iron  hood  should  be  used  for 
its  protection. 

8.  The  conductors  from  the  controller  may  be  carried  in 
conduit  to  the  motor,  and  the  mo'tor,  conduit  and  controller 


Forward  Reverse 

12345678  57  6£W  I 


13 


"  H)  Connections  of  drake 
Coils  when  Used.  If  Brake 


Series. 


CONTROLLER    CONNECTIONS. 


FIG.  244. — Wiring  connections  for  a  drum  type  controller. 

should  be  well  connected  to  a  good  ground  pipe  to  protect  the 
operator  against  sneak  currents. 

9.  The  different  points  or  stops  on  the  controller  are  deter- 
mined by  a  notched  star  wheel  just  underneath  the  top  cover. 
If  its  spring  is  broken  or  very  weak,  provide  the  good  tension 
needed  for  positive  operation,  as  placing  the  drum  between 
contacts  will  cause  trouble. 

.10.  If  the  motor  fails  to  run  when  the  controller  is  on 
the  second  or  third  point,  shut  it  off  and  test  for  current  at  the 
line  contacts  with  a  test  lamp.  Further  testing  of  the  motor 


.  -. 


358         ARMATURE  WINDING  AND  MOTOR  REPAIR 

primary  connections  at  the  controller  contacts  may  reveal  an 
open  circuit.  If  the  motor  still  refuses  to  run,  test  the  resist- 
ance for  open  circuit  and  look  carefully  over  the  slip-ring 
brushes  for  poor  contacts.  Exposed  wiring  between  the  motor, 
resistance  and  controller  may  get  broken  and  thus  stop  its 
operation  One  open  slip  ring  connection  will  cause  slow 
speed. 

11.  The  sudden  and  repeated  Operations  to  which  a  con- 
troller is  subjected 'demands  that  it  be  tightly  bolted  down. 
For  this  purpose  cast  lugs  are  provided  to  take  the  bolts. 

12.  To  keep  dust  from  getting  in  and  sparks  from  getting 
out,  the  front  case  should  be  kept  fastened  on  by  the  finger 
nuts  provided  on  both  sides. 

13.  The  control  of  the  speed  is  accomplished  by  means  of 
the  stepped  segments  which  cut  out  the  resistance  gradually. 

14.  The  ON  and  OFF  positions  should  be  plainly  marked 
for  the  operator's  guidance. 

15.  See  that  a  limit  stop  is  provided  either  in  the  shape  of  a 
cast  projection  or  an  extra  long  tooth  on  the  star  wheel,  so  that 
the  segments  can  not  be  rotated  beyond  a  whole  circle. 

16.  It  is  advisable  to  provide  a  circuit  breaker  in  the  line 
with  the  controller  so  that  in  case  it  is  left  on  part  speed  with  a 
dead  line,  no  damage  can  be  done  to  either  the  motor  or  its 
driven  machine  when  the  current  returns. 

17.  One    phase    connection    open  will  prevent  the  motor 
from  starting,  and  it  may  smoke  while  trying  to  start. 

IV.   OVERHAULING  A  LARGE  COMPOUND  D.-C.  MOTOR 

All  motors  should  be  inspected  and  given  a  general  over- 
hauling at  least  once  a  year.  Large  sizes  should  not  be  slighted 
in  any  detail  just  because  they  look  rugged  and  appear  to  stand 
everlasting  work.  Close  observation  of  all  parts  and  con- 
nections will  often  reveal  some  surprising  defects.  However 
small,  they  should  not  be  allowed  to  go  unattended. 

When  repairing  a  motor,  it  is  advisable  to  remove  all  fuses 
and  put  them  into  the  tool  kit  or  your  pocket  so  that  by  no 
chance  current  can  be  switched  onto  the  motor  when  not 
expected. 


INSPECTION  OF  MOTORS  AND  GENERATORS 


359 


In  case  of  a  belt  drive,  shift  the  motor,  slip  off  the  belt  and 
tie  the  latter  up  to  something  near  by.  If  direct-connected, 
open  the  coupling  by  removing  all  the  bolts.  Replace  the 
nuts  on  them  and  tie  the  whole  bunch  together  to  save  losing 
them. 

If  the  motor  is  suspended,  it  would  be  well  to  lower  it  with 
two  cham  tackles  and  disassemble  it  on  the  floor,  as  its  parts 
are  too  heavy  to  carry  down  a  ladder  and  rather  large  to  step 
over  on  a  scaffold.  If  on  a  shelf  or  platform,  slinging  it  to 
the  floor  is  often  not  very  difficult. 


-© 


© 


-@ 


FIG.  245. — Essential  parts  of  a  compound  direct-current  motor.     (Figures 
refer  to  number  of  paragraphs  of  text.) 

1  *.  Remove  the  key  and  pulley,  first  marking  or  measuring 
the  latter's  location,-  which  will  help  in  replacing  it.  If 
direct-connected,  remove  the  commutator  end  shield  to  allow 
the  armature  shaft  to  draw  open  the  coupling.  Mark  location 
of  the  coupling  on  the  shaft.  If  the  key  is  very  tight,  remove 
the  set  screws  and  pour  kerosene  into  the  holes,  which  will 
work  around  the  key,  then  some  motor  oil  may  be  dropped 
in  to  lubricate  the  key-way.  By  lightly  tapping  proper  shaped 
drifts,  the  key  can  often  be  started  in  a  few  minutes  without 
a  lot  of  battering  and  damage.  When  the  key  is  removed  if 
the  pulley  is  tight  on  the  shaft,  more  kerosene  will  help. 
Also  by  using  an  18-inch  monkey  wrench  on  the  rim,  it 

*  The  following  paragraph  numbers  refer  to  the  parts  shown  in  Fig.  245. 


360         ARMATURE  WINDING  AND  MOTOR  REPAIR 

can  often  be  started.  Do  not  hammer  on  the  rim,  it  may 
crack.  If  made  of  paper  it  will  flatten.  Strike  a  hard  wood 
block  butted  against  the  hub  or  drive  thin  wedges  behind.  Tie 
the  key  and  set  screws  to  the  pulley  when  removed  for  security. 

2.  Before   removing   commutator   end    shield,    mark   the 
position  of  the  rocker  arm  which  may  get  shifted. 

3.  With  the  pulley  removed,  try  to  lift  the  shaft  a  trifle 
in  the  bearing  lining  to  learn  if  it  is  worn  out  of  true.     A  year's 
wear  may  cause  the  armature  to  strike  the  lower  pole  shoe. 
It  is  well  to  observe  this  before  disassembling. 

4.  Mark  or  tag  the  armature  leads  connected  to  the  brush 
rigging,  then  disconnect  and  remove  the  brushes. 

5.  Drain  the  oil  wells  and  remove  both  end  housings  also 
the  brush  rigging.     Close  up  the  oil  wells  before  the  screw 
plugs  get  mislaid.     Be  careful  of  the  glass  gauges. 

6.  Remove  the   armature  to  a  pile  of  blocking  straight 
ahead.     This  can  be  done  with  one  lift  by  slipping  a  large 
iron  pipe  over  the  pulley  shaft,  which  will  carry  the  armature 
clear  through   and   two  feet  beyond   without   dropping  it. 
A  suspended  motor  can  be  disassembled  to  best  advantage 
on  the  floor.     Set  the  armature  on  some  burlap  and  not 
where  any  metal  clips  will  imbed  themselves  into  the  insula- 
tion.    Clean   the  armature  thoroughly,   especially  the   air- 
ducts. 

7.  If  the  commutator  has  enough  stock  left  and  is  rough  or 
grooved,  turn  it  down  in  a  lathe,  taking  only  very  fine  cuts 
with  a  diamond  pointed  tool.     Then  polish  with  oil  and  No.  00 
sandpaper,    but   not   emery.     If   the"  commutator  is   badly 
worn  down,  a  new  one  should  be  on  hand  and  replaced,  having 
the  same  number  of  segments. 

8.  Blow  the  dust  out  of  the  field  frame  and  from  around  the 
field  coils,  or  scrape  with  a  stick  but  use  no  metal  bar  or  knife. 
Wipe  out  clean  with  waste  and  some  benzine.     Examine  the 
insulation  of  the  field  coils,  shunt  and  series  field  leads,  and  if 
any  abrasion  is  found  use  tape,  thick  armature  varnish  or 
shellac  for  insulation.     If  the  field  coils  are  loose,  tighten 
the  bolts  usually  found  outside  the  frame.     If  a  coil  is  burned 
out,  remove  and  re- wind      Weigh  the  wire  that  is  stripped  off 
and  replace  the  same  size  and  weight  carefully  wound  on  and 


INSPECTION  OF  MOTORS  AND  GENERATORS         361 

well  insulated.  In  replacing  a  field  coil,  its  shunt  or  series 
polarity  are  liable  to  be  uncertain.  To  determine  this, 
properly  connect  to  the  field  windings  three  or  four  cells  of 
dry  battery  and  with  a  magnetic  compass  explore  the  polarity 
of  each  pole,  marking  it  with  chalk  for  reference.  For  both 
the  series  and  the  shunt  windings,  the  two  markings  should  be 
similar  on  any  one  pole,  then  the  fields  coincide.  The  newly 
wound  coil  should  have  an  opposite  sign  to  the  coil  on  either 
side;  if  not,  change  its  connections  and  test  repeatedly  to 
make  sure,  for  the  operation  of  the  motor  depends  on  the 
field  polarities.  Use  armature  varnish  or  shellac  on  all 
coils  and  leads. 

9.  Do  not  paint  the  inner  surface  of  pole  shoes. 

10.  If  the  commutator  leads  are  loose  solder  solidly  and 
neatly,    being    careful    not    to    let   hot    solder    drop    down 
behind  the  commutator  as  it  is  liable  to  cause  a  short  circuit. 

11.  If  the  armature  has  been  striking  the  pole  shoes,  the 
banding  wires  will  likely  be  polished  in  spots  and  partly  worn 
through.     Even  if  they  are  only  loose,  repair  them  now  by 
slipping  under  plenty  of  shellaced  mica. 

12.  If  any  armature  coil  is  burned  out,  slip  in  a  new  one  and 
thus  put  it  in  good  shape  with  full  power. 

13.  Caliper  the  armature  shaft  at  both  ends.     If  badly 
worn  or  cut  a  new  one  should  be  fitted  in  so  that  new  bearing 
linings  will  fit  perfectly. 

14.  If  the  bearing  linings  are  worn  enough  to  allow  loose 
play  of  the  shaft,  put  new  ones  in  the  end  shields.     Extra 
linings  should  be  kept  in  stock  the  year  round  as  it  will  save 
hours  delay  when  one  wears  out  suddenly  from  poor  oil 
feed.     Remove  the  set  screw  and  strike  against  a  hard  wood 
block  on  the  outer  end  of  the  lining  but  avoid  damaging  the  oil 
rings.     Usually  each  end  has  a  different  sized  lining  so  that  the 
proper  pair  are  needed.     If  the  oil  rings  get  damaged  true 
them  at  once.     Try  the  linings  on  the  shaft  to  see  if  they  fit 
the  bore.     If  linings  are  split  they  are  easily  removed.     Fit 
the  new  ones  exactly  into  place  under  their  set  screws  no 
matter  how  long  it  takes,  for  if  a  lining  ever  turns  over  out 
of  place  it  will  run  hot  and  be  ruined.     If  the  old  linings  are 
re-babbitted  on  the  premises,  have  oil  grooves  and  channels 


362         ARMATURE  WINDING  AND  MOTOR  REPAIR 

cut  in  for  lubrication  and  scrape  the  insides  for  a  perfectly 
snug  fit. 

15.  Flush  out  the  oil  wells  with  gasoline  or  benzine  and  wipe 
them  out  dry  with  cheese  cloth,  as  waste  is  liable  to  leave 
threads  to  entangle  the  oil  rings. 

16.  Clean  the  brush  rigging  especially  around  the  insulators 
on  the  holder  bars.     Renew  short  brushes  and  bevel  them 
as  near  as  possible.     Renew  broken  or  weak  tension  springs. 
See  that  the  holder  bars  are  bolted  tightly  or  they  may  turn. 
Clean  out  the  brush  holders  and  allow  the  brushes  free  move- 
ment, with  the  pig-tails  fastened  tightly  for  good  current 
connection. 

17.  Now  replace  the  brush  rigging,  put  in  the  armature  and 
put  on  both  end  shields.     Connect  the  leads  to  the  brushes. 
Set  the  rocker  arm  in  its  original  neutral  position  and  oil  the 
bearings  enough  to  allow  the  shaft  to  be  rotated  by  hand.     If 
it  binds,  look  for  the  cause  at  once,  as  it  should  turn  easily 
with  the  oil  rings  in  their  grooves.     Turn  the  armature  by 
using  a  monkey  wrench  on  the  shaft  pushing  against  the  key 
covered  with  tape  to  avoid  marring. 

18.  Set  the  brush  holders  one-eighth  inch  above  the  com- 
mutator.    Bevel  all  new  and  old  brushes  by  drawing  under 
them  strips  of  No.  1  sandpaper  until  a  good  and  full  contact 
surface  is  assured.     Have  the  tension  on  all  springs  similar, 
but  not  too  strong.     Smear  the  commutator  with  a  little 
vaseline;  it  will  not  carbonize  like  oil.     Replace  the  pulley 
and  key  and  then  rotate  the  armature  and  see  that  no  pig-tails 
interfere. 

19.  Replace  the  motor  and  bolt  it  down  firmly.     Oil  the 
bearings  fully  and  run  the  armature  with  current  for  a  few 
minutes  with  the  belt  off.     Look  sharply  for  trouble  especially 
see  that  the  oil  rings  are  lubricated  properly. 

20.  Slip  on  the  belt,  tighten  it  up,  and  run  motor  under  some 
load.     If  direct-connected  have  the  coupling  well  insulated 
and  securely  bolted.     Look  especially  for  sparking  as  the 
brushes  may  have  imperfect  contact,  too  light  spring  tension 
or  be  off  neutral.     Watch  the  motor  for  a  few  hours  underload. 
Motor  brushes  should  be  set  back  of  neutral  point,  or  given  a 
"lag"  as  it  is  termed. 


INSPECTION  OF  MOTORS  AND  GENERATORS         363 

21.  If  a  compound  motor  persists  in  sparking  badly  under 
nearly  a  full  load,  it  may  be  that  during  the  repairing  process 
the  shunt  and  series  fields  were  connected  opposed  to  each 
other,  whereas  their  polarities  should  be  similar  at  each  pole. 
Opposed  fields  really  cause  a  still  weaker  field  at  increased 
loads  and  will  result  in  sparking  and  flashing,  but  with  the 
series  .and  shunt  fields  magnetized  in  harmony  there  should  be 
no  sparking  at  the  brushes  if  they  have  good  full  contacts  and 
are  neutrally  placed. 

22.  If  the  direction  of  the  armature  is  wrong,  reverse  the 
brush  leads. 

V.    OVERHAULING    A    60-HORSEPOWER    INDUCTION    MOTOR 

Although  an  induction  motor  does  not  frequently  call  for  a 
complete  overhauling,  the  details  given  in  what  follows  cover 
such  a  case  in  order  to  bring  out  the  point  which  should  be 
considered  when  the  motor  is  completely  taken  apart  and  then 
reassembled  after  repairs. 

1*.  Remove  all  fuses  and  kill  the  line. 

2.  Slip  off  the  belt  and  try  turning  the  rotor  by  hand.     If 
it  will  not  turn,  then  one  trouble  has  been  located. 

3.  Remove  the  key  and  pulley  by  driving  wedges  behind  the 
latter. 

4.  Drain  the  oil  wells  and  remove  both  end  shields  which  in 
this  size  of  motor  are  probably  in  half  sections. 

5.  Remove  the  rotor  straight  out  ahead  onto  a  pile  of  block- 
ing.    Slip  a  pipe  over  the  short  shaft  so  that  the  rotor  may  be 
carried  through  with  one  lift.     If  it  seems  to  be  polished  on 
its  periphery,  this  is  evidence  that  at  least  one  bearing  lining 
is  worn  badly  and  has  allowed  the  rotor  to  drop.     Clean  the 
rotor  of  all  dust  and  dirt. 

6.  Clean  the  primary  winding  with  waste  and  benzine  and 
then  look  for  any  possible  abrasion  on  the  winding.     If  nec- 
essary apply  some  thick  shellac.     Coat  all  the  winding  with 
armature  varnish,  which  will  dry  in  a  short  time. 

7.  Remove  both  bearing  linings  if  they  are  worn  enough  to 
allow  loose  play  on  the  shaft.     If  they  are  split  they  may  be 

*  The  following  paragraph  numbers  refer  to  the  parts  shown  in  Fig.  246. 


364         ARMATURE  WINDING  AND  MOTOR  REPAIR 


re-babbitted  on  the  premises,  but  must  fit  snugly  and  have 
oil  channels  properly  cut  in  them. 

8.  Clean  out  both  oil  wells  as  dirt  is  liable  to  settle  and  stop 
the  rings. 

9.  Replace  the  rotor  and  oil  the  bearings.     Try  to  turn  the 
former  by  hand,  and  see  that  the  oil  rings  turn  freely. 

10.  If  the  motor  seems  to  be  in  good  shape  put  on  the  pulley 
and  key  it  securely. 


Relay  Plunger  - 

'SetScre*- 

White  Mark,  .... 

Calibrating  Point. 

Piston  Rod  •• 


Overload 
Calibrating 
"-Scale  ^ 


Dash  Pot 
OVERLOAD    RELAY 


ROTOR 

FIG.  246. — Squirrel  cage  induction  motor  installation  showing  arrange- 
ment of  starter  and  type  of  relay  that  can  be  used  instead  of  fuses.  (Figures 
refer  to  numbers  of  paragraphs  of  text.) 

11.  Open  the  compensator  case  and  look  for  trouble.     Most 
compensator  cases  after  being  unbolted  should  be  let  down  care- 
fully as  they  contain  oil  in  which  the  contacts  are  submerged. 

12.  Pull  the  operating  handle  over  to  the  starting  side  and 
see  that  the  contact  fingers  are  making  good  connections. 
If  any  are  burned  short  renew  them.     If  their  cables  or  leads 
are  not  all  tight,  one  or  more  may  be  found  which  has  the  con- 
ductor broken  inside  of  the  insulation.     These  must  be  re- 


INSPECTION -OF  MOTORS  AND  GENERATORS        365 

connected  solidly.  Now  throw  the  operating  handle  sharply 
to  the  running  side  and  examine  the  other  contact  fingers  and 
their  connections.  After  all  repairs  are  made  to  the  fingers 
and  contacts  replace  the  oil  case  and  put  in  the  fuses  for  a  trial 
spin  of  the  motor.  It  ought  to  run  nicely  now. 

13.  Try  the  no- voltage  release  by  opening  the  switch.     If 
it  fails  to  work  instantly  see  if  the  solenoid  core  is  free  to  move. 
If  dirt  is  obstructing  it  or  if  the  rod  is  bent,  remedy  it  and 
adjust   the   stop   nut   for   a  certain  distance.     See  that  the 
solenoid  leads  are  properly  connected  to  one  phase  of  the 
motor  line  behind  the  compensator.     This  circuit  energizes 
the  no-voltage  release  coil. 

14.  See  that  the  fuses  fit  snugly  in  the  clips,  as  a  loose  fit 
is  liable  to  disconnect  one  phase.     See  that  the  line  wires  are 
tightly  connected  to  the  cutout  and  switch.     Loose  connec- 
tions on  a  50-hp.  line  are  liable  to  heat  very  quickly.     The 
fuses  for  a  50-hp.  compensating  starter  ought  to  be  not  over 
150-amp.  size  with  knife-blade  clips.     This  allows  for  about 
20  per  cent,  overload  at  starting.     If  there  is  much  load  on 
the  motor  when  being  started,  the  fuses  may  be  increased  to 
175  amp.,  but  if  they  are  too  large  there  is  no  overload  protec- 
tion afforded  to  the  motor. 

15.  For  a  motor  of  this  size,  it  is  worth  the  expense  of 
installing  overload  relays  in  place  of  fuses,  as  they  can  be 
time-set  for   overloads   of   dangerous   duration.     The   small 
spring  contact  on  top  of  each  relay  is  connected  in  series  with 
the  no-voltage  release  at  the  side  of  the  compensator  and  also 
in  series  with  one  phase  from  the  motor.     Thus  an  overload 
on  the  motor  raises  the  relay  plunger  which  opens  one  or  both 
of  the  top  contacts,  which  in  turn  kills  the  no-voltage  release 
solenoid  and  by  its  gravity  drop  the  controlling  handle  is 
tripped  to  dead  center  thus  cutting  off  the  power.     The  coil 
of  each  overload  relay  is  connected  in  series  with  each  phase 
of  the  motor  so  that  both  are  protected.     The  oil  dash  pot 
needs   to   be   filled   with   relay   oil   and  screwed  up  tightly. 
The  plunger  may  be  set  at  any  amperage  and  locked  with  a  set 
screw.     In  the  piston  cup  is  a  plate  having  a  few  holes  through 
which  the  oil  is  forced  when  the  plunger  raises.     This  plate 
may  be  set  for  any  time,  either  slow  or  fast.     It  is  well  not 


366         ARMATURE  WINDING  AND  MOTOR  REPAIR 

to  have  the  plunger  work  too  quickly  as  an  induction  motor 
may  be  called  upon  to  carry  a  heavy  load  for  about  a  minute, 
when  it  would  be  bothersome  to  have  the  relays  kick.  But 
more  than  a  minute  allows  a  large  current  to  heat  up  the 
motor.  These  relays  are  trustworthy,  and  when  encased  in  a 
steel  box  they  work  silently  and  promptly  just  when  con- 
ditions demand  their  action.  When  they  kick  out  and  the 
power  goes  off,  the  relays  will  reset  themselves,  but  they  will 
act  again  at  the  very  next  overload. 

After  all  the  operations  outlined  in  items  1  to  15  have  been 
carefully  completed,  the  job  may  be  considered  finished 
and  the  motor  ready  to  run. 

VI.    OVERHAULING  A  26-HORSEPOWER  SLIP-RING  MOTOR 

A  machine  like  a  bull-dozer  in  a  forging  shop,  a  crane  or  a 
dredge  is  likely  to  have  a  motor  of  the  slip-ring  type  with  an 
external  resistance  control.  Such  a  motor  draws  current  from 
the  line  with  no  heavy  rushes  and  will  start  with  not  more  than 
one  and  one-quarter  times  its  normal  full  load  demand.  This 
is  a  good  feature  as  it  is  easy  on  the  generator,  fuses  and  line 
voltage.  Otherwise  there  might  be  the  annoyance  of  stopping 
and  starting  every  few  minutes.  These  motors,  however, 
need  repairing  at  times  like  all  other  motors  and  the  following 
procedure  is  given  for  such  repair : 

1*.  Assume  a  case  that  upon  removing  the  belt  and  trying 
to  turn  the  rotor  by  power,  it  is  found  that  it  runs  very  slowly, 
suddenly  blowing  a  fuse  or  the  overload  release  opens  the 
line  and  the  motor  stops. 

2.  Remove  all  fuses,  drain  the  oil  wells  and  remove  both  end 
shields. 

3.  Remove  the  bearing  linings  from  the  end  shields  and 
try  them  on  the  shaft.     If  they  are  worn  loose  renew  or  re- 
babbitt  them  to  fit  snugly,  and  cut  in  oil  grooves. 

4.  Remove  the  armature  and  rest  the  shaft  on  some  block- 
ing.    Examine  the  slip  rings  and  if  they  are  worn  down  thin 
or  rough,  either  renew  them  or  turn  them  down  very  smoothly 
in  a  lathe.     They  are  liable  to  roughen  from  poor  brush 

*  The  following  paragraph  numbers  refer  to  the  parts  shown  in  Fig.  247. 


INSPECTION  OF  MOTORS  AND  GENERATORS         367 


contact.  If  renewed  see  that  they  are  properly  insulated 
from  each  other  with  one-quarter-inch  fiber  board,  and  tightly 
bolted  up. 

5.  See  that  the  band  wires  are  tight  on  the  armature.     If 
they  are  worn  thin  renew  and  solder  them  all  around. 

6.  Coat   all   the   armature   winding  with   a  special  black 
varnish,  avoiding  the  core  disks,  for  magnetic  conduction. 


BRU5H    RIGGING. 


25-HP.   SLIP  RING   MOTOR. 


( Leads  -to  Armature 
'(Winding  Secondary. 


WIRING. 

FIG.  247. — Parts  and  external  resistance  control  for  a  slip  ring  alternating- 
current  motor.     (Figures  refer  to  numbers  of  paragraphs  of  text.) 

7.  Clean  all  the  dust  and  dirt  from  out  of  the  primary 
winding,  but  use  no  sharp  metal  as  the  insulation  is  easily 
damaged.     Wash    with    benzine,    and    coat   with   armature 
varnish. 

8.  Clean  the  brush  rigging  of  all  metal  dust  and  gummed  oil. 
Renew  the  short  brushes  and  the  broken  and  weak  tension 
springs.     All  brushes  should  have  good  sized  pig-tails  to  carry 
the  current.     The  brush  contacts  should  be  perfect  on  the  ring 
surfaces. 


368         ARMATURE  WINDING  AND  MOTOR  REPAIR 

9.  Replace  the  armature,  brush  rigging  and  both  end  shields. 
Smear  vaseline  on  the  slip  rings  and  adjust  the  tension  springs 
fairly  strong. 

10.  See  that  the  leads  to  the  brushes  are  not  loose  or  dis- 
connected as^  the  armature  will  turn  very  slowly  and   heat 
badly  if  only  one  conductor  is  open.     It  makes  no  difference 
to  which  ring  any  lead  is  connected  coming  from  the  external 
resistance. 

11.  Examine  the  rheostat  and  see  that  each  lead  is   con- 
tinuous from  its  own  resistance  section  and   that  no    coil 
is  open  or  loosely  connected.     The  sliding  arms  must  bear 
simultaneously  on   successive  buttons.     If  any  are  worn  or 
badly  burned  they  should  be  renewed. 

12.  The  motor  primary  leads  must  be  connected  each  to  the 
proper  leg  of  the  three-  or  four- wire  supply  line. 

13.  See  that  the  line  is  securely  bolted  to  th^  switch,  and  have 
the  latter's  clips  clean  and  capable  of  a  tight  contact,  as  cur- 
rent for  a  25-hp.  motor  is  liable  to  burn  any  loose  connections. 

14.  Overload  relays  or  fuses  may  be  used  with  this  motor. 
Relays  save  time  as  they  can  be  re-set,  and  limited  to  any 
reasonable  load. 

15.  Oil  the  bearings  fully  and  run  the  motor.     See  that  the 
oil  rings  are  turning  properly. 

16.  When  operating  a  slip-ring  motor  be  careful  to  move 
the  rheostat  lever  slowly  to  avoid  a  rush  of  current.     For 
speed   regulation   the   controller   has  many  speeds  forward 
and  reverse,  with  twenty  or  more  contact  fingers  and  blocks 
which  have  to  be  renewed  at  times  because  of  arcing. 

17.  If  a  non-reversing  motor  runs  the  wrong  way,  merely 
exchange  the  leads  of  one  phase  of  its  line.     Changing  the  slip- 
ring  connections  has  no  effect. 

18.  A  slip-ring  motor  should  have  its  brushes  and  rings 
protected  from  dust  and  dirt,  and  the  wear  on  the  bearings 
watched  just  as  closely  as  those  of  an  ordinary  induction  motor. 

VII.    OVERHAULING  A  SINGLE-PHASE  COMMUTATOR  MOTOR 

On  account  of  the  rather  special  construction  of  the  single- 
phase  commutator  motor,  there  are  certain  points  to  which 
the  repairman  should  pay  especial  attention  when  it  becomes 


INSPECTION  OF  MOTORS  AND  GENERATORS 


369 


necessary  to  overhaul  such  a  motor  or  make  repairs  to  it. 
These  points  are  outlined  in  detail  in  the  following  paragraphs 
which  refer  to  numbers  of  Fig.  248. 

1.*  Laminated  field  frame  of  the  motor. 

2.  Main  field  winding  which  produces  the  main  flux.  The 
auxiliary  field  regulates  the  power  factor  and  is  connected 
permanently  to  one  auxiliary  brush,  having  a  switch  contact 


FIG.  248. — Essential  parts  of  a  single-phase  commutator  motor.     (Figures 
refer  to  numbers  of  paragraphs  of  text.) 


for  the  other  auxiliary  brush.  Both  of  these  windings  should 
be  coated  with  black  armature  varnish  to  improve  their 
insulation. 

3.  Commutator  end  shield. 

4.  Oil-well  cover.     If  this  cover  is  missing,  a  new  one  should 

*  The  following  paragraph  numbers  refer  to  the  parts  shown  in  Fig.  248. 
24 


370         ARMATURE  WINDING  AND  MOTOR  REPAIR 

be  fitted  to  keep  dust  from  settling  where  it  may  cause  the  oil 
ring  to  stop. 

5.  Oil  gauge.     When  overhauling  a  motor  take  apart  and 
clean  out  the  gauges  as  any  obstruction  may  produce  a  false 
oil  level.     The  bearings  should  also  be  drained  and  flushed 
with  gasoline. 

6.  Oil  plugs.     See  that  they  are  tight  or  the  oil  may  drip 
from  them  and  waste. 

7.  Brush-holder  studs.     Bolt  all  of  these  up  tight  to  avoid 
tipping  the  brushes  from  the  commutator  by  their  accidental 
turning. 

8.  Carbon   brushes  with  copper  pig-tails,  in  the  variable 
speed  compensated  type  of  single-phase  motor,  remain  on  the 
commutator   all  the  time,  similar  to  direct-current  motors. 
With  the  repulsion  type  they  are  short-circuited  at  the  start 
but  are  removed  by  a  centrifugal  governor  when  up  to  speed. 
Give  all  brushes  good  contact  with  the  commutator  by  drawing 
under  them  No.  00  sandpaper  against  the  carbon.     A  little 
vaseline  will  stop  the  squeak  of  brushes. 

9.  Short-circuiting  connection  on  the  back  of  the  brush 
yoke  for  the  main  brush  set  or  energy  brushes. 

10.  Brush  holders.     See  that  they  are  cleaned  of  all  gum 
and  dirt  to  allow  free  action  of  the  brushes.     Renew  weak 
springs. 

11.  The  brush  yoke  is  sometimes  of  moulded  composition 
and  sometimes  of  cast  iron.     This  should  be  well  cleaned  to 
avoid  short  circuits. 

12.  A  fan  is  used  to  keep  up  a  circulation  of  cool  air  against 
the  windings. 

13.  Laminated  armature  with  slot  windings.     The  squirrel- 
cage  winding  is  usually  placed  in  the  bottom  of  the  slots,  and 
the  compensating  winding  near  the  surface,  and  between  the 
two  windings  a  steel  bar  separator.     All  coils  should  be  well 
insulated  to  be  moisture  proof.     See  that  band  wires  are 
tightly  soldered,  with  plenty  of  mica  under  them.     If  the 
bearing  linings  are  worn,  the  surface  of  the  armature  will 
show  polished  places. 

14.  Commutator.     To  it  are  connected  the  leads  of  the 
commutated  type  of  armature  winding.     In  the  repulsion 


INSPECTION  OF  MOTORS  AND  GENERATORS         371 

type  the  commutator  bars  are  all  connected  at  full  speed  and 
the  armature  runs  like  an  induction  rotor.  See  that  all  com- 
mutator leads  are  well  soldered  in.  If  commutator  is  rough, 
turn  it  down  in  a  lathe,  but  ever  so  little  at  a  time  with  a 
diamond-point  tool.  Polish  with  No.  00  sandpaper  and  oil. 
Never  use  emery  cloth. 

15.  Shaft.  In  some  makes  if  it  becomes  worn  or  damaged 
it  can  be  easily  withdrawn  and  a  new  one  replaced.  If  its 
bearing  surfaces  are  cut  they  should  be  turned  down  smoothly 
before  new  bearing  linings  are  fitted  on. 

16  and  17.  Motor  terminals  to  the  line.  Some  designs  allow 
for  110-volt  operation  by  connecting  the  terminals  in  parallel, 
and  220-volt  operation  with  them  in  series. 

18.  Terminals  from   the   auxiliary  or  compensating  field 
winding. 

19.  Pulley  end  shield. 

20.  Bearing  linings.     If  they  are  worn  enough  to  allow  loose 
play  of  the  shaft,  new  ones  should  be  put  in  or  the  old  ones 
re-babbitted  with  oil  channels  cut  in  from  end  to  end. 

21.  Oil  rings.     See  that  they  are  not  bent  out  of  shape  and 
when  re-assembling  be  careful  to  get  them  into  the  slots. 
This  is  very  important  and  a  light  might  better  be  used 
than  to   guess   at   their  location.     Also   turn   the  armature 
by  hand  and  see  that  the  rings  turn  also.     Oil  well  before 
running. 

22.  Pulley  should  be  tightly  keyed  on  and  fastened  with  a 
set  screw.     Only  a  balanced  motor  pulley  should  be  used. 

23.  Set  screw  for  motor  pulley. 

NOTE. — No  rheostat  is  needed,  although  speed  controllers  are 
used.  To  reverse  the  armature  rotation,  interchange  the 
leads  to  the  compensating  or  auxiliary  brushes. 

VIII.   OVERHAULING  A  DIRECT-CURRENT  ENGINE  TYPE 
GENERATOR 

At  least  once  a  year  the  generators  of  any  manufacturing 
plant  should  be  thoroughly  overhauled,  cleaned,  painted,  and 
minor  repairs  made,  even  though  not  badly  out  of  order. 
Such  an  inspection  does  no  harm  and  often  reveals  a  weak 
part  that  may  later  cause  a  break-down  at  a  busy  time. 


372 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


A  good  job  can  be  done  in  two  or  three  days  by  two  good 
repairmen  on  machines  up  to  150  kilowatts. 

1 .  *  If  belt  driven  mark  the  position  of  the  sliding  base  before 
setting  back  with  the  belt  screw. 

2.  On    both    belt-driven    and    direct-connected   machines, 
mark  the   position  of  the  rocker  arm,   assuring  the  return 
of  the  brushes  to  the  neutral  point. 

3.  Avoid  if  possible,  disturbing  the  brush  holders. 


-© 


FIG.  249. — Parts  of  an  engine  type  direct-current  generator  that  should  be 
regularly  inspected.     (Figures  refer  to  paragraphs  of  the  text.) 

4.  With  a  traveling  crane  or  a  tripod  rigging,  remove  the 
outboard  bearing  and  pedestal,  then  the  whole  brush  rigging 
"en  masse."     Be   careful  to  mark  the  armature  leads  for 
proper  re-connection. 

5.  The  armature  shaft  should  be  blocked  up  level  to  its 
bearing  before  the  latter  is  removed,  otherwise  the  poles  will 
have  to   support  the  weight  and  the  other  bearing  sleeve 
may  crack. 

*  The  following  paragraph  numbers  refer  to  the  parts  shown  in  Fig. 
249. 


INSPECTION  OF  MOTORS  AND  GENERATORS         373 

6.  Remove  either  the  pulley  or  the  coupling  key  from  the 
other  end  of  the  shaft,  and  by  properly  slinging  the  armature 
it  can  be  withdrawn  far  enough  out  past  the  poles  to  allow 
any  small  work  to  be  done.     No  hitch  or  sling  should  be  made 
around  the  commutator,  even  if  it  is  covered  with  burlap. 
Support  all  weight  by  the  shaft  only. 

7.  Blocking  under  the  armature  core  should  be  ready  to 
support  all  of  the  weight  when  withdrawn,  and  so  arranged  as 
to  prevent  its  rolling  sideways. 

8.  Now  with  either  an  air  hose,  hand  bellows  or  brush 
remove  all  dust,  dirt,  and  lint  from  around  the  poles,  in  the 
air  ducts,  and  from  inside  the  frame,  first  covering  the  engine 
crank,   if  direct  connected.     The  armature  winding  should 
be  blown  out  neatly,  and  where  any  dirt  is  inclined  to  adhere 
use  a  narrow  flat  stick  to  avoid  damaging  the  insulation. 

9.  After  a  good  dusting  all  around  a  washing  with  gasoline 
will  remove  any  remaining  gummy  dirt  or  oil. 

10.  Black  armature  varnish  may  be  applied  to  all  windings 
and  leads,  but  not  to  the  pole  shoes,  or  armature  core  disks, 
because  the   distance  between  such   parts  is  usually  small 
and   too   much  varnish   may   "freeze"   the  armature  tight 
against  the  lower  poles  after  a  shut-down. 

11.  The  armature  winding  and  leads  may  be  painted  up 
to  the  commutator  bars  but  not  beyond.     Plenty  of  it  between 
the   commutator  open  leads  helps  to  insulate  them.     The 
band  winding  at  the  outer  end  of  the  commutator  and  its 
bolted  end  plate  may  also  be  painted. 

12.  If  the  commutator  is  very  rough  or  badly  cut  it  should 
be  turned  down  either  in  a  lathe  or  by  a  turning  tool  attached 
to  the  generator  frame.     After  turning  off  ever  so  little  copper 
No.  00  sandpaper  should  be  applied  with  oil  for  a  polish, 
but  use  no  emery.     If  the  commutator  is  badly  worn  down  and 
beyond  repair,  a  new  one  should  be  on  hand,  having  been 
ordered  weeks  in  advance,  giving  the  manufacturer  all  the 
specifications  possible  from  the  name  plate.     Furthermore, 
when  the  commutator  arrives  examine  the  bore,   diameter 
length  and  number  of  segments,  otherwise  somebody's  mis- 
take may  not  be  discovered  till  the  machine  is  taken  apart. 
Do  not  pound  on  the  commutator  bars  as  they  may  sink. 


374          ARMATURE  WINDING  AND  MOTOR  REPAIR 

13.  If  the  shaft  shows  any  marks  of  cutting  have  its  surface 
trued  before  any  bearing  sleeve  is  repaired. 

14.  If  the  bearing  sleeves  are  worn  decidedly,  have  them 
re-babbitted  to  fit  snugly  but  not  too  tightly,  and  grooved 
for  oiling. 

15.  Be  careful  not  to  damage  the  oil  rings,  especially  when 
replacing  the  bearings.     Also  see  that  all  sediment  is  removed 
from  the  oil  wells,  and  the  glass  gauges  cleaned  out. 

16.  If   the   armature    band  wires  or  mica  insulation  are 
loose,  slip  under  some  sheets  of  mica  and  re-solder  the  bands 
securely. 

17.  See  that  no  field  coils  are  loose  on  the  poles;  if  so  apply 
more  wedging  of  wood  strips. 

18.  If  the  brushes  are  dirty  remove  them,  one  set  at  a  time, 
to  avoid  any  mix  up  and  clean  the  holders  with  gasoline,  allow- 
ing the  brushes  to  move  freely.     Replace  short  brushes  with 
new  ones;  renew  weak  springs;  tighten  the  pig- tails;  or  if  of 
copper  gauze  they  may  be  set  ahead.     Give  each  brush  the 
proper  and  same  tension,  not  too  loose  and  not  so  tight  as  to 
cause  cutting.     Allow  about  one-eighth  inch  under  the  holders, 
and   tip  brushes  at   proper  angles.     Get  the   proper  bevel 
on  each  brush  to  fit  the  commutator  arc  by  drawing  No.  0 
or  No.  1  sandpaper  under  it,  cutting  the  carbon  away  while 
it  rests  on  the  commutator.     Vaseline  applied  lightly  to  the 
commutator  will  help  the  lubrication  of  the  brush  contacts 
and  prevent  cutting  and  sparking. 

19.  Connect  the  main  leads  to  their  proper  bolts  tightly, 
and  if  belt  driven  slip  on  the  pulley  with  its  key  and  set  screw, 
and  with  plenty  of  oil  in  the  bearings  turn  the  armature  by 
hand  a  number  of  times  and  watch  all  parts  for  interference. 
Especially  see  that  the  oil  rings  are  turning. 

20.  A  timely  warning — do   not   coat  the   generator  with 
aluminum  paint  just  for  looks,  as  it  is  a  metallic  conductor. 
Leaky  voltage  and  shorts  may  result.     White  enamel  radiates 
heat   the   best,    although   it  is  some  trouble  to  keep  clean. 
Dark  green  is  the  usual  color. 

A.  All  these  operations  take  some  time  when  properly  done, 
but  what  a  pretty  running  machine  one  has  when  they  are 
honestly  completed. 


INSPECTION  OF  MOTORS  AND  GENERATORS         375 

B.  If  any  insulation  is  found  peeled  off  or  cracked  open, 
use  a  little  mica,  tape,  shellac,  or  armature  varnish. 

C.  When  ready  to  assemble  the  generator,   put  all  bolts 
possible  into  place  before  tightening  any.     Close  all  oil  cocks 
and  fill  up  the  bearings.     Keep  the  armature  leads  away  from 
the  rotating  armature. 

D.  When  all  is  ready,  slip  on  the  belt,  slide  back  the  base 
to  its  former  position  and  start  the  engine  slowly,  watching 
carefully  the  oil  rings. 


CHAPTER  XV 
DIAGNOSIS  OF  MOTOR  AND  GENERATOR  TROUBLES 

The  electric  motor  and  generator  are  sturdy  pieces  of  ap- 
paratus when  given  the  proper  attention  and  operated  within 
their  normal  ratings.  The  troubles  which  they  develop  can 
usually  be  .traced  to  two  main  causes.  1.  Faulty  operating 
conditions.  2.  Mechanical  and  electrical  defects. 

Faulty  operating  conditions  may  either  bring  to  light  exist- 
ing mechanical  and  electrical  defects  or  create  new  ones. 
Under  normal  conditions,  therefore,  correct  operating  con- 
ditions will  reduce  to  a  minimum,  the  troubles  due  to  the 
second  cause.  The  factors  in  faulty  operating  conditions  may 
be  classified  as  follows: 

1.  Lack  of  proper  cleaning. 

2.  Operation  in  damp  places. 

3.  Exposure  to  acid  fumes  and  gases. 

4.  Lack  of  frequent  inspection  and  replacement  of  worn 
parts. 

5.  Operating  temperatures  too  high. 

Lack  of  Proper  Cleaning. — All  machinery  having  moving 
parts  requires  lubrication  and  however  well  the  provisions  are 
made  to  confine  the  oil  to  the  parts  needing  the  lubrication,  it 
will  find  its  way  in  time  to  adjacent  parts  of  the  machine. 
In  motors  and  generators  the  oil  which  is  allowed  to  accumu- 
late on  the  windings  has  a  detrimental  effect  on  the  quality 
of  the  insulating  materials  used  and  is  a  frequent  cause  of 
short  circuits  and  grounds.  The  presence  of  oil  also  invites 
the  accumulation  of  dust  and  dirt  which  quickly  fills  up  small 
spaces  that  have  been  provided  for  ventilation  of  windings 
and  core.  This  results  in  an  increase  in  temperature  of  the 
parts  involved.  The  only  remedy  for  troubles  resulting  from 
this  cause  on  a  repaired  machine,  or  when  a  new  one  is  sub- 
stituted, is  frequent  cleaning  at  regular  intervals.  Motors 


MOTOR  AND  GENERATOR  TROUBLES  377 

operated  in  especially  dusty  places,  as  in  cement  and  flour 
mills,  should  be  blown  out  daily  with  compressed  air  or  a 
hand  bellows.  Care  must  be  taken  not  to  blow  the  dust  into 
the  windings.  An  air  pressure  of  not  more  than  50  pounds 
per  square  inch  with  a  J^-inch  nozzle  should  be  used  to  avoid 
injury  to  insulation. 

Operation  in  Damp  Places. — The  materials  employed  in 
the  insulation  of  windings  of  motors  and  generators  consist  of 
special  papers,  cotton  tapes  and  cloth,  and  mica.  With 
the  exception  of  mica,  all  of  these  materials  absorb  moisture 
more  or  less  or  are  said  to  be  hydroscopic.  In  damp  places 
where  the  operation  of  the  machine  is  not  continuous  enough 
for  the  generated  heat  to  keep  moisture  out,  the  insulation 
may  absorb  enough  moisture  to  reduce  the  insulating  strength 
to  the  rupture  point  resulting  in  all  sorts  of  troubles.  Motors 
and  generators  should  not  be  installed  in  damp  places  except 
they  be  provided  with  windings  having  a  special  moisture- 
proof  insulation  which  manufacturers  can  furnish  when  the 
conditions  under  which  the  machine  is  to  operate  are  known. 
In  case  a  machine  has  been  exposed  to  dampness  or  been 
soaked  with  water  in  a  fire  without  having  the  insulation 
damaged  by  the  fire,  it  should  be  thoroughly  dried  out  before 
being  placed  in  operation.  (For  details  of  drying  out  a  ma- 
chine, see  pages  181  to  185.) 

Exposure  to  Acid  Fumes  and  Gases. — As  in  the  cases  of  oil 
and  water,  acid  fumes  destroy  the  insulating  strength  of  most 
insulating  materials.  They  also  attack  the  metal  of  the 
machine  and  thus  bring  on  commutator  troubles.  In  chemi- 
cal works  and  in  industrial  plants  where  acid  fumes  are  present, 
small  motors  should  be  enclosed  in  a  fume-tight  case  which 
can  be  ventilated.  Where  this  is  impossible  specially  in- 
sulated windings  must  be  provided.  The  coils  needed  in  the 
repair  of  such  a  machine  should  be  secured  from  the  manufac- 
turer and  in  most  cases  it  will  be  advisable  for  the  manufac- 
turer to  do  the  repair  work.  Machines  operated  should 
be  cleaned  more  frequently  than  under  other  conditions  and 
the  exposed  windings  painted  regularly  with  an  acid-resisting 
paint  or  a  good  linseed  oil  paint.  The  commutator  should  be 
wiped  daily  with  a  little  vaseline.  The  metal  parts  of  the 


378         ARMATURE  WINDING  AND  MOTOR  REPAIR 

machine  should  also  receive  a  coat  of  acid-resisting  paint  at 
frequent  intervals. 

Lack  of  Frequent  Inspection  and  Replacement  of  Worn 
Parts. — Short  circuits  and  grounds  are  frequently  traced  to 
rubbing  of  the  motor  winding  on  field  poles  caused  by  a  bent 
shaft  or  worn  bearings.  A  worn  or  bent  shaft  may  also  cause 
the  core  of  the  armature  to  strike  the  pole  faces  and  jam  the 
punchings  of  the  core  enough  to  cut  the  coils  and  cause  a 
ground  in  the  winding.  A  loose  fit  of  the  commutator  on  the 
shaft,  when  it  has  been  replaced  after  a  repair  job,  may  result 
in  sufficient  movement  to  break  off  the  leads  to  armature  coils 
at  the  necks  of  the  commutator.  The  re-soldering  of  such  a 
defect  does  not  cure  the  trouble.  An  inspection  should  be 
made  to  locate  the  cause  and  eliminate  it.  In  this  case  it- 
will  probably  call  for  a  new  shaft.  A  mixture  of  dust  and 
foreign  material  in  the  bearing  oil  cups  or  not  enough  oil  is  the 
start  of  many  bearing  troubles  which  can  be  eliminated  by 
frequently  draining  the  bearings  and  cleaning  with  kerosene. 
When  oil  is  used  that  has  stood  for  some  time  in  a  receptacle 
or  been  pumped  from  barrels  to  a  tank,  it  is  advisable  to  filter 
it  before  using  it  in  motor  or  generator  bearings.  Wear  of 
bearings  should  be  inspected  frequently  when  a  large  tight 
belt  is  used  on  the  motor. 

Roughness  of  the  commutator  should  be  guarded  against. 
When  it  occurs  and  stoning  does  not  remedy  the  trouble,  the 
real  cause  should  be  searched  out.  The  same  applies  to  high 
mica.  The  air  gap  of  the  machine  should  also  be  frequently 
checked  by  thickness  gauges  for  this  is  a  good  check  on  the 
condition  of  the  bearings.  A  record  made  of  all  of  these  tests 
and  inspections  for  reference  when  other  troubles  develop 
will  save  much  time  and  trouble. 

A  frequent  check  for  proper  end  play  of  the  armature  shaft 
in  its  bearings  will  prevent  grooving  of  the  commutator  that 
otherwise  might  be  suspected  to  be  caused  by  unsuitable 
brushes. 

Operating  Temperatures  Too  High. — All  insulating  material 
such  as  paper  and  cotton  materials  have  a  maximum  tempera- 
ture limit  above  which  they  will  not  retain  their  insulating 
strength  and  will  simply  char  and  eventually  crumble,  causing 


MOTOR  A*ND  GENERATOR  TROUBLES  379 

electrical  faults  in  the  windings.  The  materials  which  have 
the  highest  maximum  operating  temperature  are  miea,  and 
synthetic  resins  like  bakelite.  Where  surrounding  tempera- 
tures are  known  to  be  high  as  in  the  case  of  motors  used  to 
drive  blowers  in  a  boiler  room  or  the  scrapers  on  an  economizer, 
special  heat-resisting  insulation  must  be  provided  instead  of 
that  ordinarily  used. 

For  the  ordinary  design  of  industrial  motor  the  Standardi- 
zation Rules  of  the  American  Institute  of  Electrical  Engineers 
recommend  the  following  as  maximum  temperatures  for  the 
different  kinds  of  insulating  materials  specified: 

For  cotton,  silk,  paper  and  other  fibrous  materials  not  so  treated  as  to 
increase  the  thermal  .limit,  95 °C.  (203°F.).  When  measured  by  ther- 
mometer, 80°C.  (176°F.). 

For  cotton,  silk,  paper  and  other  fibrous  material  treated  or  impreg- 
nated and  including  enameled  wire,  105°C.  (221°F.).  When  measured 
by  thermometer,  90°C.  (194°F.). 

For  mica,  asbestos,  or  other  material  capable  of  resisting  high  tempera 
tures,  in  which  the  previous  materials  if  used,  are  for  structural  purposes 
only  and  may  be  destroyed  without  impairing  the  insulating  or  mechan- 
ical qualities,  125°C.  (257°F.).  When  measured  by  thermometer,  110°C. 
(230°F.). 

All  parts  of  electrical  machinery  other  than  those  whose  temperature 
affects  the  temperature  of  the  insulating  material,  may  be  operated  at 
such  temperatures  as  may  not  be  injurious  in  any  respect.  But  no  part 
of  continuous-duty  machinery  subject  to  handling  in  operation,  such  as 
brush  rigging,  shall  have  a  temperature  in  excess  of  100°C.  (212°F.)  for 
more  than  a  very  brief  time. 

NOTE. — When  a  thermometer  is  used  to  measure  the  maximum  (hottest 
spot)  temperature,  a  correction  of  15°C.  is  added  to  the  thermometer 
reading  in  order  to  allow  of  the  impossibility  of  locating  the  thermometer 
at  the  hottest  spot,  which  will  be  beneath  the  insulation  next  to  the  metal 
of  the  conductor. 

The  hand  is  a  very  poor  thermometer  and  should  not  be 
relied  upon  except  as  a  general  indicator  of  operating  tem- 
peratures. Any  temperature  above  120°F.  is  very  uncomfor- 
table to  the  touch  but  is  well  within  the  range  of  safe  opera- 
tion. Until  a  surface  temperature  of  about  176°F.  is  reached 
there  is  little  danger  to  injury  of  insulation.  The  motor 
should  however  be  closely  watched  at  this  temperature  and 
not  allowed  to  go  much  beyond  it  except  for  a  very  short 
time. 


380          ARMATURE  WINDING  AND  MOTOR  REPAIR 
CAUSES  AND  REMEDIES  FOR  TROUBLES  IN  DIRECT-CURRENT  MACHINES 


Faults 

Cause 

How  most  readily 
detected 

Remedy 

1.  Too  high 

1.  Too     high    speed 

1.    Voltmeter    reads 

1.   Slow   the   engine. 

voltage. 

of  engine. 

greater   than  standard 

and   lamps   burn  with 

undue  brilliancy. 

2.  Too  strong  mag- 

2. Same. 

2.  Introduce    more 

netic  field. 

resistance    in    shunt 

field. 

2.  Too  low 

1.  Too  low  speed  of 

1.  Voltmeter  shows 

1.  Increase  speed  of 

voltage. 

engine. 

lower  than  standard 

engine. 

and  lamps  burn  dimly. 

2.  Too    weak    mag- 

2. Same. 

2.  T  a  k  e  out  resist- 

netic field. 

ance  in  shunt  field. 

3.  Brushes  not  prop- 

3. Same. 

3.  Rock   brushes 

erly  set. 

sack  and  forth    till 

highest  voltage  consist- 

ent  with   sparkless 

commutation  is  shown. 

3.  Excessive 

1.  In    a    generator, 

1.  By  too  high  read- 

1. Cut  out  necessary 

current. 

too  many  lamps  burn- 

ing   of    ammeter    for 

number  of  lamps.    Re- 

ing or  motors  running 

capacity  of  machine. 

duce    load    on    motor 

By  excessive  sparking 

circuits.     In  this  case 

of     dynamo     brushes 

none    of    the    motors 

and  too  high  reading 

may     be     doing     too 

of  dynamo  ammeter. 

much  work,  but  there 

may  be   too  many  in 

dynamo  circuit. 

2.  In  a  motor,    too 

2.  By     excessive 

2.  Reduce   the   load 

much    mechanical 

sparking       of     motor 

on  the  motor. 

work  being  done  by  it. 

brushes    nad  too  high 

reading    of     motor 

ammeter. 

3.  Short  circuit;  leak 

3.    By     excessive 

3.  Locate     and     re- 

or ground  in  external. 

sparking    of   brushes, 

move  leaks  or  grounds. 

circuit. 

and  heating  of  whole 

armature. 

4.  Short    circuit    in 

4.  By     heating     of 

4.  Stop    machine. 

armature  coil. 

short-circuited    coil 

Locate      coil.     If    en- 

more than  the  others. 

tirely  burned  out  must 

be  renewed. 

5.  Grounds  in  arma- 

5. Same  as  4. 

5.  Locate     the 

ture.     Two     grounds 

grounds.      Re-insulate 

to  the  core  amount  to 

the     coils     containing 

a  short  circuit. 

them. 

6.  Due  to  excessive 

6.  By    sparking    of 

6.  File     away     pole 

friction  in  bearings  or 

brushes.     By     sound 

pieces  or  re-center  arm- 

by armature  striking 

of   armature   striking 

ature.     Clean  and  oil 

pole  pieces.     In  gen- 

while     running.      By 

journals,  or  re-fit  bear- 

eral any  cause  tending 

heating  of  motor  bear- 

ings. 

to  slow  motor. 

ings. 

MOTOR  AND  GENERATOR  TROUBLES 


381 


CAUSES  AND  REMEDIES  FOR  TROUBLES  IN  DIRECT-CURRENT  MACHINES 

(Continued) 


Faults 

Cause 

How  most  readily 
detected 

Remedy 

4.  Excessive 

1.  Excessive  current; 

1.  Same  as  given  un- 

1. Same     as     given 

sparking    at 

therefore  due  to  any 

der    "Excessive    cur- 

under "Excessive  cur- 

brushes. 

of    the    causes    given 

rent." 

rent." 

under  that  head. 

2.  Brushes  improp- 

2. By  taking  brushes 

2.  Fit    and    set    ac- 

erly set. 

out  of  holders  and  ex- 

curately,    then     shift 

amining  rubbing  sur- 

the brushes    backward 

face.      By    measuring 

or  forward   till  spark- 

the    peripheral     dis- 

ing   is    reduced    to    a 

tance  between  brush 

minimum. 

sets. 

3.  Brushes    make 

3.  By   sighting   un- 

3. Sandpaper   the 

poor      contact      with 

derneath     between 

brushes  and  adjust  the 

commutator. 

brushes  and  commu- 

spring    tension     until 

tator. 

they    rest    evenly    on 

commutator  with  light 

but  even  pressure. 

4.  Rough,  non-con- 

4. A  rough  commu- 

4. Smooth    commu- 

centric commutator. 

tator  can  be  detected 

tator   with   fine  sand- 

by   lightly     touching 

paper.     If     eccentric- 

finger nail  to  it  whi'e 

ity  is  due  to   uneven 

-    ',• 

running;  an  eccentric 

wear   of  bearings,    re- 

commutator   by    the 

new  or  re-line  them. 

regular   rise   and   fall 

of  the  brushes. 

5.  "High"  or  "flat" 

5.  By   the  jumping 

5.  Same  as  above,  or 

bars  in  commutator. 

or    vibrations    of   the 

turn  down  the  commu- 

brushes. 

tator    in    lathe.     Slot 

out    the     mica    to    a 

depth  of  Ho  or  \$Q  in. 

6.  Broken  circuit  in 

6.  Commutator 

6.  Locate      coil     by 

armature  or  commu- 

flashes,   and    nearest 

drop   of   potential 

tator. 

the  break  is  cut  and 

method.      If    in    com- 

burnt.    Flashing  con- 

mutator   bridge    over 

tinues  when  armature 

the  break.     If  in  arm- 

is slowly  turned. 

ature  coil,  it  must  be 

renewed. 

7.    *Veak  fie  d  mag- 

7. Dynamo  fails  to 

7.   Short    circuits   or 

netism,    caused    by 

generate     full     emf. 

grounds  are  easily  lo- 

broken circuit  in  field 

,  If  very  weak,   motor 

cated  and  remedied  if 

winding  or  short  cir- 

runs very  slow,  taking 

external  to  the  wind- 

cuit in  same;  two  or 

a  current  many  times 

ings.      If   internal, 

more  grounds  in  wind- 

full load  current. 

faulty  coil  must  be  re- 

ings; reversal  of  one 

'•;•«• 

wound   or    repaired  if 

or  more  field  coils. 

only  grounded.     A  re- 

versed  coil  will  lower 

the  voltage  instead  of 

increasing  it,  and  it  is 

remedied  by  reversing 

the  connections. 

382 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


CAUSES  AND  REMEDIES  FOR  TROUBLES  IN  DIRECT-CURRENT  MACHINES 

(Continued) 


Faults 

Cause 

How  most  readily 
detected 

Remedy 

8.   Unequal  magnet- 

8. One  brush  sparks 

8.  Only  remedied  by 

ism. 

more  than  the  other. 

re-shaping  pole  pieces. 

9.  Dirty   commuta- 

9. Flashing    around 

9.  Clean    commuta- 

tor,   causing    brushes 

commutator. 

tor.      (Methods   given 

to  vibrate,  particular- 

later.) 

ly  if  of  carbon. 

10.   Poor   brushes, 

10.  By    ragged    ap- 

10.  Renew    brushes. 

especially  if  of  his;h- 

pearance    of    brushes 

Try  different  grades  of 

resistance   carbon, 

around  edges  and  for- 

hard and  soft  brush. 

hard  blisters  forming 

mation  of  hard  spots. 

on  them. 

11.  Vibration,  espe- 

11. By  a  humming, 

11.  Reduce  cause  of 

cially  of  brush  hold- 

singing    sound     of 

vibration   or   give   the 

ers,  causing  rapid  vi- 

brushes. 

brushes  a  little  greater 

bration  of  brushes. 

pressure  on  commuta- 

tor. 

4  (a)  Excessive 

12.  Wrong  interpole 

12.  With   low    field 

12.  In    motor,    pro- 

sparking in  inter- 

polarity. 

excitation,    examine 

gressing  in  direction  of 

pole  machines. 

field  polarity  with  a 

armature  rotation,  po- 

compass, the  armature 

larity  should  be  N-n- 

being  first  removed. 

S-s,    etc.      In    genera- 

tor, progressing  in  di- 

rection    of     armature 

rotation,     polarity 

should  be  N-s-S-n,  etc. 

13.  Interpoles     not 

13.  By  inspection. 

13.  Adjustable  when 

exactly  over  commu- 

poles are  bolted  to  the 

tation  belt. 

frame. 

14.  Brushes  not  set 

14.  Trace    out    by 

14.  Usual  setting  is 

so    that    coils    under- 

following up  coil  ends. 

in   geometric    neutral. 

going   commutation 

Set     for     minimum 

are  under  interpole. 

sparking   under   aver- 

age load. 

15.  Interpole  air  gap 

15.  See  if  all  inter- 

15. Adjustable  when 

too  long  or  too  short. 

pole  gaps  are  equal. 

poles  are  bolted  to  the 

frame.    Weaken  inter- 

pole     strength     by 

shunting  the  interpole 

winding. 

5.  Heating  of 

1.  Excessive  current 

1.  Sfeme    as    given 

1.  Same     as     given 

armature. 

through  it  and  there- 

under "Excessive  cur- 

under "Excessive  cur- 

fore due  to  any  of  the 

rent." 

rent." 

causes     given    under 

that  head. 

2.  Eddy    currents 

2.  Core     becomes 

2.  This  can  be  cor- 

and hysteresis  in  core. 

hotter  than  armature 

rected    by    improving 

coils  after  running  for 

ventilation  by  special 

a  short  time. 

fans  or  air  guides. 

MOTOR  AND  GENERATOR  TROUBLES 


383 


CAUSES  AND  REMEDIES  FOR  TROUBLES  IN  DIRECT-CURRENT  MACHINES 

(Continued) 


Faults                             Cause 

How  most  readily 
detected 

Remedy 

3.  Conduction  from 

3.  Other  parts  con- 

3. Locate  source   of 

other    parts    as    from 

nected    to    armature, 

heat  by   thermometer 

commutator  or  bear- 

as commutator,  shaft 

or   feel    by    the   hand 

ings,    the   heat   being 

or     bearings,     hotter 

and      correct  ,    it     by 

conveyed  to  armature. 

than  the  armature. 

cleaning   and  lubrica- 

tion. 

6.   Heating  of 

1.  Too    great    pres- 

1. By     feeling     the 

1.  Reduce     pressure 

commutator. 

sure  of  brushes,  fric- 

commutator with  the 

by  adjusting  spring. 

tion  causing  heat. 

hand. 

2.  Excessive  spark- 

2. Same. 

2.  Discover     the 

ing. 

cause  of  sparking  and 

correct  it  according  to 

the    particular    cause 

given  under  sparking. 

3.  Excessive     cur- 

3. Same. 

3.    Discover  cause  of 

rent. 

excessive  current  and 

correct    according    to 

particular     cause     al- 

ready given. 

4.   Conduction  from 

4.  Same. 

4.  If  from  bearings, 

other  parts. 

lubricate  orre-fitthem. 

7.   Heating  of 

1.  Excessive     cur- 

1. Too  hot  to  bear 

1.  Locate    the    par- 

field coils. 

rent    in    field    circuit 

the  hand.     If  exceed- 

ticular   coil   in    which 

due  to  short  circuits 

ingly  hot,  by  smell  of 

fault  lies  and  repair  or 

or  grounds. 

burning  shellac  or  var- 

r e-wind.     Methods 

nish  or  charring  cotton. 

given  later. 

2.  Eddy  currents  in 

2.  The    pole    pieces 

2.  Only  remedied  by 

pole  pieces,  heat  being 

are    hotter    than    the 

better  design,  use  lam- 

conducted to  the  coils. 

coils  after  a  short  run. 

inated  pole  shoes. 

8.  Heating  of 

1.  Lack   of  lubrica- 

1. By    feeling    with 

1.  Fill  oil  cups;  clean 

bearings. 

tion. 

hand.     Oil  cups  emp- 

feeding pipes. 

ty    or    feeding    pipes 

clogged. 

2.  Dirty    or    gritty 

2.  By   feeling   with 

2.  Remove  cap  and 

bearings. 

hand. 

thoroughly  clean. 

3.  Bearings    out    of 

3.  Unequal  wear  of 

3.   Bearings  must  be 

line. 

bearings,     and    shaft 

lined  up  or  shells  re- 

will  not  turn  freely  by 

babbitted.     If   very 

hand. 

serious,    new  bearings 

will  have  to  be  made. 

4.   Rough      or     cut 

4.  Shaft    will    show 

4.  Turn  down  shaft 

shaft. 

the  roughness  in  the 

in  lathe,  or  scrape  the 

bearings. 

bearings. 

5.  Shaft  bent. 

5.  Unequal  wear  in 

5.  Shafts    can    only 

bearings    and    arma- 

be    straightened     by 

ture  will  wobble.  Very 

disconnecting    from 

hard  to  move  by  hand. 

armature  and  re-heat- 

ing«and  re-forging. 

384 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


CAUSES  AND  REMEDIES  FOR  TROUBLES  IN  DIRECT-CURRENT  MACHINES 

(Continued) 


Faults 

Cause 

How  most  readily 
detected 

Remedy 

6.  Oil  rings  stuck. 

6.  Inspection. 

6.   Adjust     rings     in 

grooves. 

9.  Too     low 

1.  Too  much  load. 

1.  By  speed  indica- 

1. Reduce    the    me- 

speed   (referring 

tor;   heavy   sparking, 

chanical  load. 

to  motors). 

heating    of    all    parts 

and  bearings. 

2.  Any  of  the  causes 

2.   Same,   and  same 

2.  Discover  particu- 

given under  "  Heating 

as  given  under  "  Heat- 

lar cause  and  remedy 

of  bearings,"  causing 

ing  of  bearings." 

same   as   given    under 

excessive  friction. 

"  Heating  of  bearings." 

3.   Short    circuit    or 

3.  By  motor  taking 

3.  Same  as  under  5, 

grounds  in  armature. 

excessive     current 

"Excessive   current." 

without  load  as  shown 

by  ammeter  or  heavy 

sparking  and  heating. 

4.  Too  low  voltage 

4.   By    motor    volt- 

4. By  increasing  the 

at  terminals. 

meter  or  speed  indi- 

line voltage. 

cator.      By     heavy 

sparking  and  heating. 

10.  Too    high 

1.  Too  light  load  (in 

1.  By  noticeable  in- 

1. Increase  load. 

speed   (referring 

series  motors). 

crease  in  speed. 

to  motors). 

2.  Weak  field  shunt 

2.  Same. 

2.  Strengthen  field. 

motor. 

3.  Too  high  voltage 

3.  Same. 

3.  Correct  line   vol- 

at terminals,   due  to 

tage  by  remedies  1  and 

high  voltage  of  dyna- 

2   under    "Too    high 

mo. 

voltage." 

11.  Dynamo 

1.  Too  weak  residual 

1.  Very  little  attrac- 

1. Send     a     current 

fails  to  generate 

magnetism,  caused  by 

tion  by  the  pole  pieces 

through    field   from   a 

emf. 

a   jar    or   reversal   of 

when    tested    with    a 

few    cells    or    from    a 

current  not  sufficient 

piece  of  iron. 

running  dynamo. 

to  reverse  magnetism. 

2.  Short   circuit 

2.  Magnetism  very 

2.  Locate     the 

within     machine,     or 

,  weak. 

grounds  or  short  cir- 

grounds in  field  wind- 

cuits and  correct  them. 

ings. 

3.  Reversed     field 

3.  All  poles  should 

3.  Make  polarity  op- 

coils. 

have    alternate    mag- 

posite by  reversing  the 

netism;  if  a  coil  is  re- 

connections of  the  coil. 

versed    it    will    show 

Each   pole   should   be 

magnetism,  but  may 

opposite  to  the  one  on 

not  be  of  opposite  po- 

each side  of  it. 

larity. 

4.  Series  and  shunt 

4.  Voltage   falls    as 

4.  Reverse     connec- 

windings connected  up 

load  is  increased,  the 

tions   of  either    field, 

opposite  to  each  other 

external  circuit  being 

but  not  both 

closed    showing    that 

they     are     working 

• 

against  one  another. 

MOTOR  AND  GENERATOR  TROUBLES 


385 


CAUSES  AND  REMEDIES  FOR  TROUBLES  IN  DIRECT-CURRENT  MACHINES 

(Continued) 


Faults 

Cause 

How  most  readily 
detected 

Remedy 

5.  Brushes  not  prop- 

5. Magnetism     and 

5.   Find  central  posi- 

erly placed. 

emf.      increased      by 

tion  by  experiment  or 

shifting  the  brushes. 

from  drawings  of  con- 

nections1. 

6.  Open    circuit    in 

6.  Test  circuits  with 

6.    Set     up     on     all 

field      or      armature. 

magneto. 

connections.      Press 

Brushes    not    making 

brushes  on   commuta- 

good contact  with  the 

tor  to   start    building 

commutator.       Loose 

up. 

connections. 

7.  Too  much  resist- 

7. Voltage  does  not 

7.  Cut  all  resistance 

ance  in  the  shunt  field 

exceed    that    due  to 

out  of  the  shunt  field 

circuit,     i.e.,     greater 

residual     magnetism. 

circuit.      Reverse    the 

than  the  "  critical  "  re- 

The   voltage    due    to 

shunt  field. 

sistance.     Shunt  field 

residual    magnetism 

bucks     the     residual 

drops  when  the  shunt 

magnetism. 

field  circuit  is  closed. 

12.  Motor  fails 

1.  Too  much  load. 

1.  No    motion    and 

1.  If  motor  does  not 

to  start. 

fuse  in   circuit   melts 

start  at  once,  turn  off 

or  circuit-breaker  acts. 

current  and  search  for 

See  if  motor  runs  all 

cause.     Reduce     load 

right  when  light. 

on  motor. 

2.  Excessive  friction, 

2.  Same,  and  motor 

2.   Remedies  same  as 

due  to  any  causes  giv- 

hard to  turn  when  not 

given  under  "  Heating 

en    under    heading 

loaded,   and   with  no 

of  bearings." 

"Heating     of     bear- 

current. 

ings." 

3.  Short    circuit    of 

3.   Motor  refuses  to 

3.  If  connections  are 

field  or  armature    or 

revolve,  though  shows 

made    wrong,    consult 

among  connections. 

signs  of  strong  mag- 

maker's diagram   and 

netism.     Will   turn 

correct  them.    Test  for 

easily  by  hand  if  un- 

continuity   and    short 

loaded    and    with    no 

circuits  as  given  later. 

current.     If  current  is 

very  great,  it  is  indi- 

cation of  short  circuit. 

If    fault    is    in    field, 

magnetism     will     be 

weak. 

4.  Open  circuits  due 

4.  Weak  magnetism 

4.  Turn  current  from 

to  field  switch  open, 

shows  a  loose  connec- 

motor, and  search  for 

fuse  melted,  loose  or 

tion  in  field  circuit;  no 

cause  of  discontinuity; 

broken     connections, 

magnetism,  that  field 

examine      all      switch 

or  some  fault  at  gen- 

switch is  open.     May 

fuses  and  connections, 

erator. 

be   heavy   current  in 

tightening     all.     Test 

armature.     If  there  is 

for  continuity  in  ma- 

no  armature   current 

chine  circuits  and  re- 

there will  be  no  spark 

pair  broken  or  burnt- 

at     brushes     when 

out  coils. 

raised. 

2.1 


386 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


CAUSES  AND  REMEDIES  FOR  TROUBLES  IN  DIRECT-CURRENT  MACHINES 

(Concluded) 


Faults 


Cause 


How  most  readily 
detected 


Remedy 


13.  Flickering 
of  lamps. 


14.  Noise. 


1.  Uneven    running 
of    engine,    probably 
due  to  governor  fail- 
ing to  properly  func- 
tion. 

2.  Loose     connec- 
tions,  either   on   ma- 
chine, switchboard  or 
external  circuit. 

1.  See    third    fault 
(6),  fourth  fault  (4,  5, 
9,  10,  11),  eighth  fault 
(3,   4,   5),   and  tenth 
fault. 

2.  Armature  running 
against  the  brushes. 


1.  By  flickering  of 
lamps  or  vibration  of 
voltmeter  indicator. 


2.  Same. 


2.  By  unusual  noise. 


1.  Overhaul   engine, 
specially  governor. 


2.  Examine  all  con- 
nections and  see  that 
they  are  firm  and 
make  good  contact. 
Look  for  arcs. 


2.  Correct    direction 
of  rotation. 


Electrical  Defects. — Troubles  in  motors  and  generators 
due  to  electrical  defects  show  up  in  a  variety  of  Ways.  Their 
location,  however,  is  more  often  a  simple  than  a  difficult  matter 
and  depends  upon  a  few  testing  devices  and  a  great  deal  of 
patience  and  persistence.  An  experienced  operator  or  re- 
pairman learns  to  diagnose  electrical  troubles  by  a  process 
of  elimination  where  there  •  may  be  several  causes.  For 
such  an  analysis  of  common  electrical  troubles  in  motors 
and  generators  the  table  on  pages  380  to  386  will  be  found 
useful.  It  was  compiled  by  the  authors  of  the  Naval  Elec- 
tricians Text  Book  and  has  been  taken  from  that  most  practical 
work. 

CAUSES  AND  REMEDIES  FOR  TROUBLES  IN    ALTERNATING- 
CURRENT  MACHINES 

Induction  Motor  Troubles. — The  most  common  troubles 
with  induction  motors  show  up  by  the  machine  failing  to  start 
or  by  stopping  while  connected  to  the  line.  When  the  ma- 
chine refuses  to  start  and  the  fuses  are  not  blown,  the  cause 
may  be  in  too  large  a  load.  The  remedy  in  such  cases  is  to  either 
reduce  the  load  or  use  a  clutch  for  starting  since  the  squirrel- 
cage  motor  has  a  limited  starting  torque  but  can  often  carry 


MOTOR  *AND  GENERATOR  TROUBLES  387 

a  much  larger  load  while  running.  When  investigating  a 
trouble  of  this  kind  it  is  important  to  see  that  the  voltage  is 
normal  and  that  the  bearings  are  in  good  condition.  If  none 
of  these  conditions  seem  to  cause  the  trouble  a  test  should  be 
made  for  open  circuits  in  the  windings.  (See  Chapter  IX.) 

An  excessive  current  at  starting  may  be  due  to  a  high  volt- 
age applied  or  too  great  a  load.  When  the  voltage  is  high 
an  auto-transformer  with  suitable  taps  must  be  used  for 
starting.  When  an  auto-transformer  is  used  and  the  starting 
current  is  excessive,  a  test  should  be  made  for  proper  connec- 
tions. Too  low  taps  on  the  starter  should  be  avoided. 

An  overload  on  an  induction  motor  will  usually  show  up 
by  the  mo  tor  coming  to  rest  or  taking  about  10  times  more  than 
normal  current.  When  fuses  and  circuit  breakers  do  not 
operate  under  such  heavy  current,  the  machine  will  burn  out. 
It  is  therefore  important  that  fuses  and  settings  of  circuit 
breakers  be  used  to  prevent  such  burn-outs.  Since  the  torque 
of  an  induction  motor  varies  with  the  square  of  the  voltage, 
low  supply  voltage  may  be  the  cause  of  the  machine  stop- 
ping. Also  worn  bearings  will  allow  the  rotor  to  drop  on  the 
stator  and  thus  block  the  former  and  stop  the  machine.  For 
other  details  in  testing  and  inspection  of  induction  motors  see 
Chapter  XIV. 

Locating  Troubles  in  Winding  of  Induction  Motors. — 
When  the  cause  of  trouble  with  an  induction  motor  in  starting 
or  in  operation  has  been  traced  to  the  stator  windings,  details 
of  locating  the  fault  are  outlined  in  Chapter  IX.  When 
the  stator  has  been  repaired  and  again  placed  in  service  its  con- 
nections should  be  carefully  looked  over.  The  points  to  be 
observed  when  inspecting  such  a  stator  are  outlined  on  page 
363.  In  case  the  connections  of  an  auto-starter  are  reversed, 
the  starting  current  will  be  excessive  and  insufficient  torque 
with  the  switches  in  the  running  position  will  show  up.  If 
the  motor  refuses  to  start  the  cause  may  be  due  to  a  defect  in 
the  change-over  switch  or  to  a  loose  connection  or  open 
circuit  in  the  auto-starter. 

Mechanical  Adjustments. — In  those  cases  where  induction 
motors  are  in  continuous  operation  regular  inspections  should 
be  made  of  the  air  gap  to  see  that  it  is  not  appreciably  reduced 


388          ARMATURE  WINDING  AND  MOTOR  REPAIR 

at  the  bottom  of  the  rotor  on  account  of  wear  of  the  bearings. 
When  the  air  gap  is  reduced  a  test  should  be  made  before  the 
trouble  is  attributed  definitely  to  wear  of  the  bearings.  It  is 
also  important  that  an  armature  have  a  small  play  endwise  in 
the  bearings.  This  should  be  about  Me  inch.  Excessive 
heating  or  excessive  belt  tension  may  be  the  cause  of  a  great- 
deal  of  bearing  trouble  on  an  induction  motor  which  is  belt- 
driven. 

Troubles  Due  to  Electrical  Faults. — When  errors  have  been 
made  in  winding  an  induction  motor  or  in  re-connecting  the 
coils,  trouble  will  usually  show  up  in  heating  of  the  coils,  a 
peculiar  humming  sound  for  large  and  unbalanced  currents 
in  the  different  phases.  Before  testing  for  faults  of  re-connec- 
tion of  coils,  it  must  be  determined  that  the  connections  of  the 
winding  to  the  supply  circuit  are  correct  and  that  none  of  the 
connections  of  the  phaso  coils  have  been  accidentally  jammed 
together  so  as  to  short-circuit  them.  Troubles  in  rotors  of 
induction  motors  are  rare,  however,  in  squirrel-cage  types 
trouble  may  be  located  in  soldered  joints  which  have  melted 
or  corroded.  High  resistance  at  joints  lowers  the  efficiency, 
increases  the  heating  and  also  increases  the  starting  torque  of 
the  motor.  Heating  or  operation  at  reduced  output  or  speed 
may  be  due  to  a  three-phase  motor  running  on  one  or  two 
phases. 

Troubles  with  Synchronous  Motors. — When  a  synchronous 
motor  fails  to  start  the  trouble  is  generally  due  to  an  overload. 
In  testing  out  for  such  a  condition  the  motor  should  be  started 
light.  If  the  operation  is  not  satisfactory  the  load  should  be 
reduced.  Poor  starting  may  also  be  caused  by  reduced  volt- 
age or  open  or  faulty  connections  in  starting  apparatus.  An 
open  circuit  is  usually  indicated  by  no  current  flowing  in  a 
particular  phase.  An  excessive  current  usually  indicates  a 
short  circuit  but  may  be  due  to  grounds.  An  excessive  cur- 
rent in  a  synchronous  motor  is  a  dangerous  condition  and  the 
trouble  should  be  located. 

When  a  synchronous  motor  is  used  for  power-factor  correc- 
tion overheating  indicates  an  excessive  current.  The  machine 
can  usually  be  temporarily  operated  by  reducing  the  load  or 
reducing  the  amount  of  leading  current.  When  a  synchronous 


MOTOR  AND  GENERATOR  TROUBLES  389 

motor  is  operating  satisfactorily  the  current  in  the  armature 
phases  should  be  about  equal  when  the  rotor  is  turning  slowly. 

Trouble  in  the  field  winding  such  as  an  open  circuit  causes  a 
shutdown  or  excessive  armature  heating.  When  the  field  cur- 
rent seems  excessive,  a  test  should  be  made  for  polarity  of  the 
armature  coils  and  reversal  of  connections.  When  a  syn- 
chronous motor  fails  to  show  normal  starting  torque  and  will 
not  carry  the  load,  the  trouble  will  frequently  be  found  in  the 
field  circuit  in  the  form  of  an  open  circuit,  short  circuit  or 
reversal  of  one  or  more  field  windings. 

Causes  of  A.-C.  Motor  Fuses  Blowing. — In  addition  to 
overload  on  a  motor,  many  other  things  cause  fuses  to  blow. 
The  following  causes  and  symptoms  have  been  formulated  by 
Henry  W.  Zeuner,  Milwaukee  (Wis.)  Electric  Light  Company 
(Electrical  World,  Sept.  20,  1919). 

1.  Operator    throwing    starting    switch    of    compensators 
from  starting  to  running  position  too  quickly. 

2.  Operator  throwing  switch  into  running  position  without 
touching  the  starting  position  at  all. 

3.  Motor  winding  becoming  grounded. 

4.  Excessive    current    due    to    low    voltage,  short  circuits 
in  stator  windings,  single-phase  operation,  etc. 

5.  Starting  switch  being  in  running  position  when  service 
comes  back  on  line  after  interruption. 

Wound-rotor  motors  and  squirrel-cage  motors  which  are 
not  protected  by  a  no-voltage  release  may  be  shut  down  at 
any  time  because  of  the  last-mentioned  cause.  To  overcome 
this  trouble  a  group  of  such  motors  is  often  protected  by  an 
oil  switch  equipped  with  no-voltage  release.  The  arrange- 
ment has  the  disadvantage,  however,  that  whenever  the  oil 
switch  opens  it  becomes  necessary  for  some  one  to  go  around 
and  open  each  individual  motor  switch  before  the  oil  switch 
is  closed  again. 

A  ground  may  only  blow  one  fuse  ajid  leave  the  motor 
operating  from  one  phase  of  the  line.  When  a  polyphase 
motor  is  running  single-phase  it  not  only  gets  hot  but  it 
makes  a  growling  noise  which  is  especially  noticeable  under 
heavy  load.  By  the  time  these  symptoms  indicate  the  prob- 


390         ARMATURE  WINDING  AND  MOTOR  REPAIR 

able  source  of  the  trouble  and  it  is  decided  to  shut  down  the 
motor,  a  second  fuse  is  sometimes  blown  by  the  excessive 
current  per  phase.  If  only  the  first  defective  fuse  found  is 
replaced,  the  motor  may  be  started  from  the  line  side  of  the 
switch  which  is  connected  ahead  of  the  fuses.  When  the 
switch  is  thrown  over  into  the  running  position  the  motor  is 
again  operating  on  single  phase.  Because  the  motor  con- 
tinues to  heat  up  the  user  often  thinks  there  must  be 
something  wrong  with  the  motor  itself  and  calls  for  help, 
whereas  if  he  had  only  tested  all  the  fuses  in  the  first  place  and 
replaced  all  of  the  burnt-out  fuses,  instead  of  only  one  of 
them,  he  could  have  put  the  motor  back  in  service  without 
assistance  and  with  a  minimum  loss  of  time.  Another  source 
of  trouble  which  may  be  detected  at  the  service  fuses  is  low 
voltage.  This  causes  excessive  heating  and  often  burns 
out  stator  coils.  The  voltage  can  be  checked  roughly  with 
test  lamps. 

Inspection  of  Motor-starting  Devices. — From  the  fuses  it  is 
customary  to  proceed  to  examine  the  motor-starting  switch, 
compensator  or  controller.  Contact  fingers  are  often  found 
bent  or  burned  so  that  they  do  not  make  contact  and  the 
connecting  leads  are  sometimes  burned  off.  The  two-minute 
starting  resistance  supplied  by  the  manufacturers  for  starting 
slip-ring  motors  is  often  burned  out  by  being  used  for  speed- 
regulating  duty  which  requires  a  much  heavier  resistance. 

Testing  Motor  for  Grounds. — When  testing  an  alternating- 
current  motor  for  grounds,  a  magneto  or  high-voltage  testing 
transformer  should  be  used  as  the  line  voltage  will  seldom 
show  a  ground  unless  it  is  making  very  good  contact. 
National  Electrical  Code  rules  require  that  motor  frames  be 
grounded,  but  this  requirement  is  not  always  carried  out. 
When  the  motorframe  is  insulated  from  the  ground  the  motor 
can  be  kept  in  operation  with  one  phase  grounded  to  the 
motor  frame,  but  tlae  defect  should  be  remedied  at  the  first 
opportunity.  A  second  connection  between  the  stationary 
windings  and  the  motor  frame  will  burn  out  the  coils  which  it 
short  circuits,  and  if  not  given  prompt  attention  the  two 
grounds  may  result  in  the  burning  out  of  the  entire  winding. 


MOTOR  AND  GENERATOR  TROUBLES  391 

When  both  the  supply  system  and  the  motor  frames  are 
grounded,  as  is  usually  the  case,  one  ground  on  the  stator 
winding  will  blow  one  of  the  motor  fuses  unless  the  fuse  is 
too  heavy,  in  which  case  the  ground  may  burn  out  some  of  the 
stator  coils.  However,  every  ground  which  blows  a  fuse 
does  not  occur  in  the  motor  winding.  If  the  motors  test 
clear,  the  wiring  leading  to  the  motor  may  be  found  grounded 
to  the  conduit. 

It  is  a  good  plan  to  run  the  defective  motor  while  testing 
when  it  is  practicable  to  do  so.  This  makes  it  possible  to 
conduct  additional  tests  and  very  often  to  observe  symptoms 
which  do  not  appear  when  the  motor  is  standing  still.  Under 
such  operating  conditions  the  speed  should  be  tested.  Low 
speed  and  inability  to  pull  the  load  are  usually  an  indication 
of  bad  connections  between  the  rotor  bars  and  the  rotor  end 
rings. 

Hot  Stator  Coils. — Another  condition  to  observe  while  the 
motor  is  operating  is  whether  the  stator  coils  are  hotter  in 
one  place  than  in  another.  This  is  one  of  the  signs  of  bearing 
trouble.  When  the  bearings  become  worn  the  stator  coils 
around  the  section  of  reduced  air  gaps  get  hotter  than  the  coils 
adjacent  to  the  section  where  the  air  gap  has  been  increased. 
If  the  bearings  are  not  renewed  before  the  rotor  begins  to 
rub  on  the  stator,  the  motor  windings  may  be  seriously  in- 
jured, especially  if  the  fuse  is  too  large  and  does  not  blow. 
When  it  is  impracticable  to  operate  the  motor  the  belt  should 
be  taken  off  and  the  bearings  examined  to  find  out  if  there 
is  too  much  play  in  them. 

Tension  of  Belts. — Much  belt  and  bearing  trouble  is  caused 
by  the  belt  being  too  tight.  This  is  especially  true  with 
vertical  belt  drives  from  squirrel-cage  motors  to  high-speed 
machines.  When  bearing  trouble  is  experienced  with  in- 
stallations of  this  description  it  is  usually  because  the  pulley 
surface  is  too  small  to  accelerate  the  driven  machine  as  fast 
as  a  squirrel-cage  motor  comes  up  to  speed,  and  consequently 
the  belt  comes  off.  In  attempting  to  keep  the  belt  on  the 
pulley  the  belt  is  usually  made  so  tight  that  the  excessive 
tension  soon  wears  down  the  bearings.  This  trouble  can 


392         ARMATURE  WINDING  AND  MOTOR  REPAIR 

sometimes  be  overcome  in  installations  of  small  motors  by 
using  larger  pulleys,  a  larger  motor,  or  by  changing  from  a  ver- 
tical belt  drive  to  a  horizontal  belt  drive  with  the  slack  side 
of  the  belt  on  top.  With  large  belted  motors  driving  high- 
speed machines  the  slip-ring  type  of  motor  should  be  used  in 
order  to  bring  the  driven  machine  up  to  speed  more  slowly. 

Troubles  in  Rotor  Windings. — A  bad  contact  in  the  rotor 
winding  is  not  so  easily  found  when  the  end  rings  are  cast 
on  to  the  end  of  the  rotor  bars.  This  cast  construction  was 
originally  adopted  to  overcome  the  bad  joints  which  developed 
with  screwed  and  soldered  connections  in  the  rotor  windings. 
The  change  has  not  entirely  eliminated  the  trouble,  however, 
because  the  molten  metal  does  not  always  unite  with  the  end 
of  the  copper  bars  when  the  end  rings  are  being  cast.  When 
this  is  t"he  case  trouble  due  to  the  poor  contact  often  develops, 
especially  if  the  motor  is  subject  to  much  dirt  and  vibration. 
With  a  little  experience  a  defective  joint  between  the  cast 
end  rings  and  the  rotor  bars  can  be  found  by  tapping  the  end 
rings  with  a  hammer  and  noting  the  difference  in  sound  at 
various  places.  A  bad  joint  in  this  type  of  construction  can 
be  repaired  by  welding  the  bar  and  end  rings  together.  Welded 
rotor  windings  have  been  very  successful,  and  it  is  unusual 
to  find  bad  joints  in  rotor  windings  which  have  been  put 
together  in  this  manner. 

Examination  of  Stator  Winding. — The  stator  winding 
should  also  be  thoroughly  examined  while  the  end  shields 
are  off  the  motor.  If  one  or  two  coils  are  burned  out,  the 
other  coils  can  in  most  instances  be  raised  out  of  the  slot  and 
new  coils  slipped  into  place  provided  that  the  coil  insulation 
is  flexible  and  in  good  condition.  Should  this  be  attempted, 
however,  after  the  insulation  on  the  coils  has  become  brittle, 
there  is  a  grave  danger  of  damaging  the  insulation  to  such  an 
extent  that  the  entire  stator  winding  may  have  to  be  renewed. 
When  the  condition  of  the  insulation  is  doubtful  and  only  one 
or  two  coils  need  replacing,  it  has  been  found  a  good  plan 
to  cut  away  the  burnt-out  coils  and  thread  new  wire  through 
the  slots  turn  by  turn,  without  disturbing  any  of  the  other  coils. 

The  damage  from  a  local  short  circuit  in  the  stator  winding 


MOTOR  AND  GENERATOR  TROUBLES  393 

can  be  limited  to  the  one  or  two  coils  affected  and  the  motor 
still  kept  temporarily  in  service  by  promptly  cutting  out 
the  damaged  coil  with  a  jumper.  However,  if  the  trouble 
is  neglected  it  will  spread  to  other  coils  in  the  same  phase,  and 
then  the  windings  of  the  other  phase  or  phases  will  also  burn 
out. 

Sparking  at  Slip  Rings. — This  trouble  in  wound-rotor  mo- 
tors occurs  when  the  tension  spring  has  not  been  reset  from 
time  to  time  to  keep  the  brushes  in  close  contact  with  the  slip 
rings.  This  is  the  most  frequent  cause  of  the  trouble,  and 
it  can  usually  be  eliminated  by  adjusting  the  tension  of  the 
spring  and  where  necessary  turning  up  the  slip  rings  and  re- 
newing the  brushes. 


CHAPTER  XVI 

METHODS   USED   BY   ELECTRICAL   REPAIRMAN   TO 
SOLVE  SPECIAL  TROUBLES 

When  called  upon  to  locate  troubles  in  electrical  apparatus 
many  electricians  and  repairmen  find  themselves  in  a  state  of 
wondering  just  what  to  do  first.  The  mystery  supposed  for 
so  long  to  be  associated  with  the  operation  of  electrical  appa- 
ratus seems  to  discourage  many  able  electrical  men  to  attempt 
to  search  out  an  electrical  trouble  when  it  seems  to  be  a  compli- 
cated one.  By  the  use  of  the  same  good  sense  that  makes 
such  men  successful  in  the  work  with  which  they  are  familiar, 
most  electrical  problems  in  repair  can  be  located  and  a  satis- 
factory remedy  applied.  In  a  serious  case  where  expert  help 
is  reasonable  and  within  easy  and  prompt  access,  it  may  not 
always  pay  for  an  inexperienced  man  to  spend  the  necessary 
time  to  handle  a  difficult  trouble  but  in  the  ordinary  run  of 
plant  operation  such  troubles  are  few.  What  is  most  needed 
is  a  little  clear  thinking  aided  by  a  few  testing  instruments. 

One  of  the  ways  by  which  a  repairman  can  develop  con- 
fidence in  his  own  ability  to  search  out  trouble  and  prepare 
himself  to  properly  diagnose  troubles,  is  to  read  and  profit 
by  the  experiences  related  by  those  who  have  worked  out 
puzzling  troubles  and  have  told  in  the  columns  of  electrical 
trade  journals  how  they  proceeded.  The  author  has  made 
a  selection  of  such  experiences  covering  a  wide  range  of  operat- 
ing troubles  which  are  liable  to  come  up  at  a  time  or  under 
circumstances  which  make  them  difficult  to  handle.  The 
details  given  will  furnish  suggestions  as  to  methods  of  pro- 
cedure in  many  other  cases  than  those  described. 

Sparking  at  Commutator  Caused  by  Poor  Belt  Joints. — In 
operating  motors  or  generators  with  laced  belts,  it  is  important 
that  the  joints  be  flexible  and  the  ends  neatly  laced  close 
together  to  present  as  nearly  as  possible  a  continuous  surface 
to  the  pulley.  Where  this  is  not  done  laced  belts  very  fre- 

394 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     395 

quently  cause  sparking  due  to  any  one  of  three  causes:  (1) 
The  laced  belt  when  passing  over  the  pulley  may  cause  a  jerk 
or  mechanical  vibration  of  the  armature  which  is  severe 
enough  to  throw  the  brushes  from  the  commutator  and  cause 
sparking  due  to  the  broken  circuit.  (2)  When  the  bearings  are 
loose  in  their  housings  or  when  the  armature  shaft  is  loose  in 
its  bearings,  the  jerk  of  the  lacing  may  pull  the  armature  to 
one  side.  This  develops  a  higher  voltage  in  a  generator  or  a 
higher  counter  electromotive  force  in  a  motor  in  the  part  of  the 
armature  which  is  pulled  closest  to  the  pole  faces.  This  will 
cause  sparking  due  to  a  pulsating  current  or  in  severe  cases 
due  to  a  short  circuit  current  between  studs  of  the  same 
polarity.  (3)  The  belt  may  slip  while  the  laced  ends  are 
passing  over  the  pulley,  which  will  permit  a  variation  in  the 
speed  of  the  armature.  This  will  cause  a  sudden  change  in 
the  current  and  may  produce  sparking  due  to  the  inductance 
of  the  armature  coils. 

Plugging  a  Commutator. — When  commutator  bars  show  a 
tendency  to  blister  and  bead  at  the  outer  ends,  " plugging" 
the  commutator  often  gives  relief.  To  do  this,  proceed  as 
follows:  Grind  all  of  the  "set"  from  the  teeth  of  a  piece  of 
hacksaw  blade.  With  this  piece  of  blade  saw  the  commutator 
side  mica  to  the  outline  indicated  at  (a)  in  the  accompanying 
diagram,  Fig.  250.  Next  drive  in  mica  wedges  or  "plugs"  so 
called,  to  entirely  fill  and 
perfectly  fit  the  grooves 
made  by  sawing.  The 
best  results  are  to  be  ob- 
tained by  loosening  the 
commutator  before  driv- 
ing in  the  Wedges,  but  FlG'  250.-Method  of  repairing  mica  seg- 
0  ;  ment  by  sawing  at  (a)  and  plugging. 

this     is     not     absolutely 

necessary.     For    other    details    of    commutator    repair    sec 

Chapter  XII. 

Knock  in  a  Motor  Armature  Due  to  Band  Wires  Being  Too 
High. — In  the  banding  of  many  types  of  direct-current  arma- 
tures, the  bands  are  laid  onto  a  layer  of  fish  paper  or  of  thin 
fiber  which  affords  smooth  bedding  for  the  band  wires  and  which 
also  provides  additional  insulation  between  the  band  wires  and 


396          ARMATURE  WINDING  AND  MOTOR  REPAIR 

the  armature  coils.  On  many  of  the  later  designs  of  armatures 
the  tension  used  in  banding  has  been  greatly  increased,  and  as 
a  further  precaution  against  the  bands  cutting  into  the  insula- 
tion of  any  coils  that  may  be  high  in  the  slot,  a  layer  of  thin 
tin  is  interposed  between  the  band  wire  and  the  insulating 
band.  That  it  may  not  be  advisable  to  adopt  this  improved 
method  on  machines  that  have  not  been  designed  for  it,  is 
illustrated  by  the  following  experience  (Electrical  Record, 
June,  1918). 

A  20-hp.  direct-current  motor  which  had  just  been  repaired 
and  shipped,  was  returned  within  a  few  days  with  the  com- 
plaint that  the  armature  "had  a  knock  in  it."  At  the  time  of 
testing  the  motor,  a  slight  vibration  had  been  noticed,  but 
leveling  of  the  motor  had  eliminated  the  vibration.  Careful 
inspection  of  the  motor  this  time,  however,  revealed  that  one 
of  the  tin  clips  of  an  end  band  had  been  striking  one  of  the 
bottom  pieces.  Investigation  disclosed  that  a  contributory 
cause  of  the  knocking  was  that  one  of  the  bearing  linings  had 
been  bored  a  little  out  of  center,  but  even  after  this  had  been 
corrected  a  piece  of  %4-inch  fiber  could  not  be  inserted  between 
the  armature  core  and  the  bottom  pole  pieces  in  certain 
positions  of  the  armature  The  real  cause  of  the  trouble  was, 
then,  that  the  layers  of  insulation  and  tin  under  the  band 
wires,  projected  the  wires  and  their  holding  clips  too  far  into 
the  air  gap.  As  the  air  gap  was  thinner  than  ordinarily  found 
on  such  motors,  the  remedy  was  to  remove  the  tin  and  install 
band  wires  of  smaller  diameter  and  with  less  tension,  and  to 
use  thin  copper  for  the  band  wire  clips.  These  changes  re- 
sulted in  a  %2-inch  air  gaP-  The  testers  were  deceived  as  to 
the  cause  of  the  vibration  because  the  tilting  of  the  machine 
floated  the  armature  to  a  position  where  the  high  clip  cleared 
the  pole  piece  it  was  striking. 

Heating  of  an  Armature  Traced  to  Poor  Soldering  of  Com- 
mutator Connections. — E.  C.  Parham  has  described  (Electrical 
Record,  March,  1919)  an  interesting  case  where  it  was  necessary 
to  replace  the  armature  of  an  exciter  that  had  been  in  opera- 
tion so  long  that  the  commutator  bars  had  worn  so  that  the 
pressure  of  the  end  rings  was  beginning  to  buckle  the  bars 
up  in  the  middle.  An  extra  armature  was  obtained  because 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     397 

the  exciter  could  not  be  spared  from  service.  This  armature 
was  installed  in  place  of  the  old  one  and  the  old  one  was 
repaired  by  re-filling  its  commutator.  About  a  year  later  a 
switchboard  short  circuit  which  started  by  a  stroke  of  light- 
ning, so  badly  burned  the  new  exciter  commutator  that  it  was 
necessary  to  take  a  cut  off  it.  The  repaired  armature  was 
taken  from  storage  and  installed  but  it  could  not  be  used, 
because  it  heated  all  over  as  soon  as  a  field  was  put  on  the 
exciter.  As  the  armature  was  the  only  equipment  part 
that  had  been  handled,  the  trouble  evidently  was  in  it.  It 
was  removed  and  the  commutator  disconnected  for  testing. 
The  commutator  was  found  to  be  perfectly  clear  but  the 
armature  leads  where  they  were  connected  to  the  commutator 
were  found  to  be  a  mass  of  short  circuits  due  to  the  criminal 
carelessness  of  the  man  who  had  done  the  soldering.  After 
picking  out  the  solder,  trimming  the  leads  and  re-connecting 
by  a  workman  who  knew  that  class  of  work,  the  operation 
was  normal. 

One  lesson  that  stands  out  very  prominently  from  this 
experience  is  this:  When  the  armature  of  an  indispensable 
machine  must  be  replaced  on  account  of  the  necessity  of 
making  repairs,  after  the  repairs  have  been  completed,  re-in- 
stall the  repaired  armature  at  once  for  test  so  that  the  armature 
that  is  known  to  be  right,  may  be  kept  as  the  spare  and  with 
a  fair  degree  of  certainty  that  it  will  be  all  right  when  oc- 
casion arises  for  using  it.  Another  lesson  is  that  every 
shop  or  station  that  pretends  to  do  connecting  and  soldering, 
should  have  access  to  a  short-circuit  test  for  locating  just 
such  trouble  as  described. 

How  a  Commutator  was  Repaired  under  Difficulties.  —  In 
an  instance  related  by  R.  L.  Hervey  (Electrical  World,  Feb. 
26,  1916)  the  commutator  of  a  generator  had  on  a  number 
of  occasions  given  trouble  because  of  the  mica  breaking  down 
between  the  segments  and  at  the  end  rings.  After  a  member 
of  the  engine-room  crew  had  failed  to  relieve  the  trouble, 
an  electrician  was  called  in.  Two  segments  were  located 
that  had  been  hot  enough  to  melt  the  solder  out  of  the  joint 
between  the  bars  and  the  risers.  This  appeared  to  be  the 
seat  of  the  trouble. 


398         ARMATURE  WINDING  AND  MOTOR  REPAIR 

Before  the  commutator  was  opened  it  was  blown  out  with 
an  air  blast  at  the  front  and  back  to  remove  the  copper  and 
carbon  dust  and  prevent  it  from  falling  into  the  commutator 
and  giving  trouble  when  the  latter  was  opened.  One  of  the 
bars  that  showed  signs  of  being  hot  had  welded  to  the  rear 
end  ring,  forming  a  ground.  As  this  bar  passed  from  one 
brush  to  the  other  the  ground  changed  from  the  positive  to  the 
negative  bar.  causing  the  ground  lamps  to  flicker.  Consider- 
able oil  had  crept  along  the  shaft 
and  found  its  way  into  the  com- 
mutator, starting  the  trouble  by 
FIG.  251. — Section  of  patched  causing  the  binder  in  the  mica  ring 

to  disintegrate.  Four  segments  were 

removed  so  that  a  patch  could  be  put  in  the  mica  ring,  as  shown 
in  Fig.  251.  The  old  ring  was  measured  for  thickness,  and 
two  pieces  of  special  ring  mica  were  put  in,  with  a  thin  piece 
of  clear  mica  covering  each  of  the  four  joints.  A  high-grade 
shellac  was  used  to  hold  the  pieces  in  place  until  the  commutator 
was  tightened  and  also  to  fill  up  the  small  openings  in  the 
mica  and  keep  oil  out. 

The  risers  used  on  this  machine  were  poorly  attached 
to  the  commutator  bars,  being  merely  soldered  in  a  shallow 
slot  in  the  center  of  the  bar.  Before  the  heated  segments 
could  be  replaced  it  was  found  to  be  nesessary  to  scrape  the 
thin  coating  of  solder  from  the  riser  and  out  of  the  slot  in 
order  to  make  a  good  joint.  If  a  current  is  passed  through  a 
soldered  joint  while  the  solder  is  hot  enough  to  flow,  the 
structure  of  the  solder  is  so  changed  that  the  resistance  of  the 
joint  is  very  much  increased  and  will  continue  to  give  trouble 
if  worked  near  its  original  carrying  capacity,  although  to  all 
appearance  a  good  joint  has  been  made. 

When  the  commutator  was  built  amber  mica  was  used 
between  the  bars.  About  every  two  years  it  has  been  neces- 
sary to  remove  a  few  pieces,  and  these  had  been  replaced  by 
micanite,  which  is  supposed  to  be  able  to  withstand  the  bad 
effects  of  oil  better  than  the  former.  At  this  time  10  pieces 
of  mica  that  showed  signs  of  pitting  were  taken  out.  Two 
pieces  were  of  the  original  amber  and  the  rest  were  micanite, 
some  of  which  had  not  been  in  service  more  than  two  years. 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES      399 

This  case  does  not,  however,  afford  a  fair  comparison  between 
the  two  grades  of  mica.  If  the  leads  had  been  poorly  soldered 
into  the  riser  or  the  riser  into  the  bar,  a  high-resistance  joint 
would  have  resulted,  causing  imperfect  commutation  with 
creepage  of  current  across  the  mica  between  bars.  This 
creepage  will  in  time  break  down  the  mica  segment  and  thus 
short-circuit  the  bars. 

In  order  to  test  for  a  short  circuit,  after  the  leads  were 
soldered  in  and  the  commutator  drawn  tight  the  engine  was 
started  and  the  field  switch  closed.  If  a  short  circuit  had 


FIG.  252. — Method  of  attaching  turning  rig  to  machine  showing  its  operation 
with  tool  upside  down. 

existed,  it  would  have  caused  one  or  more  coils  to  get  hot 
in  a  few  minutes,  but  such  a  condition  was  not  observed  in 
this  case. 

Since  it  had  been  necessary  to  turn  this  commutator  so 
often,  the  owners  had  purchased  a  turning  rig  which  was 
supposed  to  fit  this  machine.  A  common  difficulty  of  turning 
devices  is  that  they  seldom  fit  a  machine  unless  made  specially 
for  it.  In  this  case  if  the  rig  had  been  put  on  the  proper  side  of 
the  pedestal  it  would  have  been  necessary  to  leave  the  bear- 
ing cap  off  or  to  have  the  tool  considerably  above  the  center 
of  the  commutator.  In  the  latter  instance,  in  case  the  tool 
caught,  it  would  have  dug  into  the  commutator.  The  only 


400         ARMATURE  WINDING  AND  MOTOR  REPAIR 

other  way  by  which  the  rig  could  be  attached  was  to  place  it 
so  as  to  cut  with  the  tool  upside  down.  Although  the  rig 
was  not  so  steady  as  it  would  have  been  if  worked  in  the  cor- 
rect position,  no  difficulty  was  encountered. 

The  turning  rig  used  on  this  job  was  very  easily  attached. 
It  was  unnecessary  to  remove  a  single  brushholder  in  order 
to  put  it  in  place.  The  screws  at  A  in  Fig.  252  held  the  main 
part  of  the  rig  to  the  bearing  cap.  The  two  screws  B  were 
screwed  against  the  side  of  the  pedestal  to  steady  the  end  of 
the  rig.  By  loosening  the  screw  C  the  transverse-feed  length 
was  readily  adjusted.  The  end  of  the  transverse-feed  arm 
was  supported  by  the  long  screw  D  resting  on  a  brushholder 
arm.  To  prevent  any  lateral  motion  of  the  armature,  the 
screw  E  was  held  in  the  center  of  the  shaft  by  the  adjustable 
arm  F. 

Holder  for  Sandpapering  Commutators. — In  Fig.  253 
there  is  shown  a  holder  for  use  in  sandpapering  commutators 
that  makes  it  unnecessary  to  shape  a  block  to  fit  the  curved 
surface  of  the  commutator.  This  device  as  made  by  Peter 

J.  M.  Clute  (Popular 
Science  Monthly,  June, 
1919)  consists  essentially 
of  a  handle  which  is 
broadened  at  its  lower 
extremity,  and  has  two 
blocks,  2  by  2j-£  in.,  with 

an  arc  of  a  circle  on  each 

,  on,  ui      i 

face.     These   blocks  are 

pivoted,  and  will  adjust 
and  accommodate  them- 

FIG.    253. — Convenient    device    for    sand-          i 

papering  a  commutator.  selves     to     any     COrnmu- 

tator  curvature.     Two  or 

more  of  the  blocks  may  be  used,  depending  upon  the  size  of 
the  commutator. 

Use  of  a  Portable  Electric  Drill  to  Undercut  Mica  of  Com- 
mutator.— An  interesting  method  of  undercutting  a  commu- 
tator on  which  the  mica  had  come  to  the  surface  aftter  the 
commutator  had  been  turned,  has  been  devised  by  R.  H.  N. 
Lockyear,  of  Trail,  B.  C.  (Electrical  Record,  June,  1918). 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     401 

Mr.  Lockyear  describes  as  follows  the  circumstances  under 
which  the  electric  drill  was  successfully  used  for  this  purpose. 


FIG.  254. — Special   arrangement   to   use   a   small   portable   electric   drill   to 
undercut  mica  on  a  commutator. 


FIG.  255. — Special  rigging  for  the  use  of  an  electric  drill  shown  mounted  on  a 
generator  to  undercut  the  commutator  mica. 

"The   writer  had  occasion  to  undercut  a  commutator  on  a 
500-kw.    generator,    the   mica   having   come   to   the   surface. 


402         ARMATURE  WINDING  AND  MOTOR  REPAIR 

after  the  commutator  had  been  turned.  Work  had  com- 
menced by  hand  but  it  was  found  difficult  to  make  head- 
way. We  had  a  machine  for  undercutting  railway  commuta- 
tors having  a  width  of  about  three  inches.  This  machine  was 
rebuilt  with  longer  guide  rods  and  a  bracket  made  for  mounting 
it  on  the  brush  shifting  yoke  of  the  generator.  This  machine 
was  connected  with  a  flexible  shaft  but  soon  discarded  because 
of  the  vibration.  It  then  occurred  to  the  writer  to  direct 
connect  a  Van  Dorn  type  DA-OO  electric  drill,  which  was 
available,  to  the  spindle  of  a  milling  machine  and  undercut  the 
mica  with  this  equipment.  The  scheme  proved  most  effective 
and  since  that  time  we  have  used  it  on  26  generators  with  a 
great  saving  of  time,  which  is  an  important  factor  in  a  power 
plant  when  repairing  large  generators.  In  the  tool  we  employ, 
the  cutter  has  a  speed  of  1650  rpm.,  a  diameter  of  %-in.,  and  a 
thickness  of  0.003  in." 

Jerky  Operation  of  New  Commutator  Traced  to  Burred  Com- 
mutator Bars. — In  some  instances  where  it  has  been  necessary 
to  temporarily  reduce  the  speed  of  a  series  motor  and  there 
have  not  been  available  suitable  resistances  for  connecting  in 
series  with  the  armature,  the  desired  result  has  been  obtained 
by  connecting  a  resistance  across  the  brushes.  The  resistance 
so  disposed  not  only  diverts  part  of  the  line  current  from  the 
armature,  but  also  provides  an  external  path  through  which 
the  motor  armature  can  act  as  a  generator,  thereby  producing  a 
braking  effect.  The  extreme  of  this  condition  is  active  when 
a  railway  motor  flashes  over  from  brush  to  brush.  The  cur- 
rent generated  through  the  short  circuit  formed  by  the  arc 
is  so  great  that  the  suddenly  imposed  load  checks  the  speed  of 
the  armature,  the  sudden  checking  of  the  speed  constituting 
what  railway  men  call  "  bucking. "  An  action  which  is  similar 
in  kind  but  which  is  milder  in  degree,  occurs  when  an  armature 
coil  is  short-circuited.  The  coil  becomes 'a  short-circuited 
loop  moving  in  a  strong  field,  and  the  local  current  of  the  coil 
is  so  heavy  and  the  drag  imposed  by  it  so  great  that  the  re- 
mainder of  the  armature  may  be  unable  to  support  continuous 
motion.  Under  this  condition  the  armature  will  turn  in 
jerks,  because  the  resisting  drag  of  the  short-circuited  coil 
is  greater  in  some  positions  than  in  others. 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     403 

In  a  certain  case  (Electrical  Record,  June,  1918)  a  new  com- 
mutator was  installed  on  a  direct-current  armature  which  had 
operated  normally  up  to  the  time  of  removing  the  old  com- 
mutator. After  installing  the  new  commutator,  on  trying 
to  start  in  the  usual  manner,  the  armature  would  turn  very 
slowly  and  in  jerks,  and  if  the  starter  were  advanced  half  of 
its  travel  the  breaker  would  blow.  An  ammeter  was  connected 
in  series  with  the  armature,  and  the  current  was  found  to  be 
of  almost  full-load  value  on  the  first  notch  of  the  starter. 
A  bar  to  bar  test  was  made  around  the  commutator  and  nearly 
all  of  the  armature  coils  appeared  to  have  zero  resistance. 
Just  at  this  time,  full-load  current  being  in  the  armature,  which 
was  blocked  so  that  it  could  not  return,  an  explosion  accom- 
panied by  a  flash  occurred  on  the  commutator.  Investigation 
disclosed  a  burn  between  two  bars  in  the  groove  that  is 
turned  in  the  rear  end  of  a  commutator.  Close  inspection 
showed  that  the  groove  appeared  as  a  continuous  copper 
band  extending  entirely  around  the  commutator.  In  cutting 
the  groove  the  tool  had  been  allowed  to  drag  the  copper  over 
from  bar  to  bar  all  round  the  commutator.  On  clearing 
the  mica  bodies  of  the  copper  bridges,  operation  became 
normal. 

Why  Brush  Studs  Heated  on  an  Eight-pole  Machine. — The 
accompanying  diagram  shows  the  paths  of  the  current  through 
a  multiple  connected  arm- 
ature that  is  used  in  a  four- 
pole  machine,  the  two  brush 
studs  of  the  same  polarity 
being  connected  by  means 
of  copper  busses  with  ends 
strung  onto  the  studs  that 
are  to  be  connected.  The 
incoming  current  arriving 

at    brush    a  divides    among     FIG.    256.— Diagram   of  paths   of  cur- 

three  paths  in  order  to  reach  rent  through  armature* 

brush  d  to  which  the  other  end  of  the  external  circuit  is  con- 
nected. One  path,  ad,  is  directly  through  one-quarter  of  the 
armature  to  d;  the  second  path  is  through  one-quarter  of  the 
armature  to  brush  b  and  thence  by  way  of  bus  bd  to  brush  d 


404          ARMATURE  WINDING  AND  MOTOR  REPAIR 

and  out;  the  third  path  carries  as  much  current  as  the  two 
others  combined;  it  runs  through  bus  ac  to  c  where  it  divides 
equally,  half  passing  through  the  upper  right  hand  quarter  of 
the  armature  directly  to  d  and  out  and  the  other  half  through 
the  upper  left  hand  quarter  of  the  armature  to  brush  b  and 
thence  by  way  of  bd  to  d  and  out.  With  the  busses  of  negli- 
gible resistance  and  the  sets  of  brushes  equally  spaced  on  the 
commutator  all  of  the  studs  would  carry  approximately  equal 
currents. 

On  an  eight-pole  machine  there  are  more  current  ramifica- 
tions because  there  are  more  studs  and  more  busses  and,  there- 
fore, there  are  more  paths  for  the  current. 

It  was  on  such  a  machine  in  a  particular  plant,  that  two  of 
the  brush  studs  would  get  so  hot  as  to  burn  the  insulating 
bushings  by  means  of  which  the  studs  were  insulated  from  the 
yoke.  Investigation  disclosed  that  the  machine  was  abusively 
overloaded  and  that  all  studs  heated  above  normal  but  that 
only  two  got  hot  enough  to  burn  bushings.  An  inspector 
^suggested  that  the  heating  was  due  to  the  generally  poor  con- 
dition of  all  contacts  and  especially  to  the  poor  contacts  be- 
t"-een  the  stud  threads  and  the  threads  of  the  nuts  by  means  of 
which  the  busses,  the  yoke,  the  washers  and  the  studs  were 
held  together. 

Acting  on  this  theory,  V-shaped  pieces  of  copper  strap  were 
formed  and  so  installed  on  the  studs  as  to  form  a  conducting 
bridge1  from  the  outside  nut  and  washer  over  the  yoke  (with- 
out touching  the  yoke)  to  the  inside  nut  and  washer.  This 
bridge  short-circuited  all  contacts  with  which  the  threaded 
parts  of  the  studs  were  involved.  The  scheme  worked  so  well 
that  similar  bridges  were  installed  on  all  of  the  studs.  After 
giving  the  whole  stud  construction  a  good  cleaning  there  was  no 
further  trouble. 

An  Accident  Due  to  Incorrectly  Set  Brushes. — In  a  case 
where  an  isolated  plant  was  furnishing  a  meat  packer  with 
electric  power  at  110  volts  for  a  10-hp.  motor,  in  order  to 
provide  emergency  service,  connections  were  made  to  the 
central  station's  220-volt  mains.  The  higher  voltage  required 
the  installation  of  an  additional  motor  with  its  separate  starter. 
When  the  new  220-volt  motor  was  installed  no  electrical  test 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     405 

was  made  until  the  work  had  been  completed  and  the  belt 
put  on  the  pulleys.  With  all  the  machines  connected  to  the 
line  shaft,  the  motor  was  started  with  the  following  results. 
(R.  L.  Hervey,  Electrical  World,  Jan.  13,  1917).  An  operator 
observed  that  the  speed  was  excessive  and  ran  to  the  switch 
box,  missing  by  about  six  inches  a  piece  of  the  rim  of  a  36-in. 
iron  pulley;  another  piece  went  through  the  roof.  Before  the 
motor  stopped  the  line  shaft  was  bent  and  the  boxes  thrown 
out  of  line  by  the  unbalancing  of  the  pulleys.  The  dealer  who 
sold  the  motor  was  held  responsible  for  the  damage,  and  there- 
fore had  an  examination  made.  The  connections  were  found 
correct,  but  the  rocker  arm  was  loose  and  the  brushes  were 
about  60  electrical  degrees  back  of  the  neutral  position. 

Before  the  motor  was  started  the  brushes  were  set  as  closely 
to  the  neutral  as  could  be  by  tracing  out  the  armature  leads. 
The  line  and  armature  wires  were  taken  off  the  starter  to  test 
the  magnetic  switches,  which  were  found  to  be  working  prop- 
erly. The  line  wire  was  then  re-connected  and  the  shunt  field 
tested  and  found  complete  by  observing  the  flash  when  the 
switch  opened  and  by  holding  an  iron  bolt  against  the  field 
poles  to  test  the  pull  of  each  pole.  With  the  belt  off,  the 
brushes  set  approximately  correct  and  a  current  in  the  shunt- 
field,  the  armature  connection  was  made  and  the  switches 
closed,  the  motor  started  and  ran  as  nicely  as  could  be  wished 
for.  This  little  bit  of  carelessness  in  not  testing  the  motor 
before  putting  on  the  belt  caused  a  shutdown  for  seven  hours, 
an  expenditure  of  $24  in  repairs  and  endangered  the  life  of  one 
man. 

Wrong  Setting  of  Brushes  for  Direction  of  Rotation  Caused 
Motor  to  Flash. — If  line  voltage  be  applied  to  a  shunt- wound 
motor  without  using  any  starting  resistance  in  the  armature 
circuit,  the  immediate  result  is  likely  to  be  the  blowing  of  a 
fuse  or  the  opening  of  a  circuit  breaker,  because  the  armature 
resistance  is  so  low  compared  to  the  resistance  of  the  field 
winding  that  the  latter  is  unable  to  get  current  for  producing 
the  field  on  which  starting  depends.  In  the  case  of  a  small 
motor,  the  fuse  may  hold  and  the  motor  may  start  without 
demonstration  because  the  maximum  current  involved  is  small. 
A  large  motor  is  apt  to  flash  and  the  brushes  to  burn  owing  to 


406         ARMATURE  WINDING  AND  MOTOR  REPAIR 

their  being  unable  to  handle  an  abnormal  current.  Shunt- 
wound  motors  operate  better  with  their  brushes  set  a  little 
back  of  neutral  than  they  do  with  the  brushes  set  forward  of 
neutral  or  on  neutral,  because  with  a  slight  backward  shift 
field  distortion  helps  commutation. 

In  one  case  mentioned  by  E.  C.  Parham  (Electrical  Record, 
October,  1918),  a  20-hp.  motor  had  been  overhauled  and 
was  to  be  given  a  light  running  test.  As  there  was  no 
starting  box  available,  it  was  necessary  to  start  the  motor 
from  full  voltage  through  a  switch  and  also  a  circuit  breaker. 
On  closing  the  breaker  and  then  closing  the  switch,  there  was 
a  heavy  flash  at  the  brushes  followed  by  blowing  of  the  circuit 
breaker,  but  the  motor  failed  to  start.  Thinking  that  the 
flashing  might  be  due  to  full  voltage  being  applied  to  armature 
and  field  connected  in  parallel  and  without  a  starting  resist- 
ance, the  motor  was  re-connected  so  that  its  field  became 
energized  on  closing  the  breaker,  the  armature  receiving  cur- 
rent on  closing  the  switch  after  closing  the  breaker.  An 
attempt  to  start  again  caused  flashing  and  blowing  of  the 
breaker,  but  this  time  the  armature  actually  began  to  turn 
and  it  was  observed  to  have  started  to  turn  in  the  wrong 
direction.  On  reversing  the  field  connection,  there  was  no 
further  demonstration.  All  of  the  trouble  was  due  to  the 
brushes  having  a  forward  shift  when  the  motor  was  connected 
to  run  in  the  wrong  direction. 

Proper  Adjustment  of  a  Reaction-type  Brush-holder. — 
The  angle  at  which  a  reaction-type  brush-holder  is  set  on  a 
commutator  plays  an  important  part  in  the  life  of  the  brushes 
and  commutator.  When  the  brush  is  set  against  the  direction 
of  rotation  of  the  commutator,  as  shown  in  Fig.  257,  the  correct 
setting  of  the  brush  is  of  greater  importance  than  when  set  in 
the  opposite  direction.  If  the  brush  is  set  too  straight,  the 
friction  will  pull  it  away  from  the  holder  and  cause  a  chattering 
and  sparking.  If  it  is  set  too  flat,  there  will  be  a  tendency  for 
the  brush  to  wedge  between  the  holder  and  the  commutator. 
Some  machine  manufacturers  using  this  style  of  brush-holder 
furnish  a  templet  by  which  the  holder  can  be  accurately  set, 
and  in  such  a  case  it  should  be  carefully  used. 

R.  L.  Hervey  (Electrical  World,  Jan.  22,  1916)  relates  a  case 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     407 


where  he  was  called  upon  to  locate  the  cause  of  sparking  on  a 
generator  using  this  type  of  holder.  Upon  making  inquiries 
it  was  learned  that  the  engineer  had  purchased  a  sheet  of  high- 
grade  carbon,  so  called,  and  made  his  own  brushes.  To  save 
carbon  and  work,  both  ends  were  made  with  the  same  angle 
and  the  holder  was  turned  to  suit  the  brush.  The  brush 
length  had  been  made  so  great 
that  the  arm  pushed  the  brush 
away  from  the  holder  instead 
of  against  it.  The  length  L 
(Fig.  257)  should  never  be  so 
great  that  end  of  arm  B  will  be 
above  the  line  AO  drawn  per- 
pendicular to  the  face  and 
through  the  arm  center  0.  The 
manufacturers  of  this  holder 
recommend  that  the  holder  be 
set  as  shown;  nevertheless,  there 
are  a  .great  many  installations 
working  in  the  opposite  direc- 
tion. 

Heating  of  Brush-holders  Traced  to  Defective  Contact 
Springs  and  Remedied  by  a  Flexible  Shunt. — R.  L.  Hervey 
(Electrical  World,  July  1,  1916)  relates  the  following  conditions 
in  a  case  where  his  attention  was  called  to  the  discoloring  and 
heating  of  brush-holders  on  several  generators.  The  holder 
was  of  the  parallel-motion  type,  as  shown  in  the  sketch, 
Fig.  258.  To  prevent  the  burning  of  the  hinge  joints  of 
the  parallel  motion,  phosphor  bronze  springs  0.010  in.  thick 
were  used  to  carry  the  current  around  these  points.  As  long 
as  the  contact  between  these  thin  springs  and  the  moving 
arms  was  perfect  no  trouble  occurred.  But  as  soon  as  any 
part  of  the  contact  became  defective  an  overload  was  thrown 
on  the  remaining  contacts,  causing  overheating.  The  total 
contact  area  of  the  springs  on  the  holders  of  the  same  polarity 
was  0.360  sq.  in.  fo'r  250  amp.  A  properly  designed  contact  to 
carry  this  amount  of  current  should  have  1%  sq.  in.  of  surface 
To  relieve  the  trouble,  the  springs  were  taken  out  and  replaced 
with  a  flexible  shunt,  as  shown  in  the  sketch.  In  such  a  case 


FIG.  257. — Diagram  showing 
proper  setting  for  reaction-type 
brush-holder. 


408 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


it  should  be  mentioned  that  unless  the  soldering  of  the  flexible 
wire  into  the  lug  is  very  carefully  done  the  solder  will  get  into 
the  wire  and  stiffen  it.  In  order  to  prevent  this,  the  shanks 
of  the  lugs  were  flattened  with  the  wire  in  it,  forming  a  very 
strong  electrical  joint. 

The  box  type  of  construction  of  this  holder  afforded  a  sub- 
stantial contact  between  the  holder  and  the  brush.  Due  to  the 
weight  of  the  moving  box  and  its  supporting  arms,  however, 
this  brush-holder  was  displaced  by  a  stationary  holder  and 
sliding  brush  with  a  light  spring.  With  a  high  commutator 
speed,  the  heavy  brush  and  holder  does  not  follow  the  irregular- 


Woren  ff/re  Shunf-. 


Sen* 


•Spring 


FIG.  258. — Substitution  of  woven  wire  shunt  for  springs  of  parallel  motion 

brush-holder. 

ities  of  the  commutator  as  well  as  the  lighter  brush.  The 
brush  that  can  move  in  its  holder  is  better  able  to  follow  the 
grooves  of  the  commutator  as  the  armature  weaves  backward 
and  forward  in  the  bearings.  Although  the  brush-holder  is 
usually  considered  a  minor  part  of  the  motor  or  generator,  it 
has  held  the  attention  of  the  designing  engineer  for  a  good 
many  years  and  still  needs  the  attention  of  the  operator  and 
repair  man. 

Simple  Scheme  for  Banding  Armatures. — The  method 
shown  in  Fig.  259  has  been  found  convenient  in  one  repair 
shop  (G.  H.  Vescelius,  Electrical  World,  July  19,  1919,  page 
132)  where  a  banding  lathe  and  tension  device  was  not  avail- 
able and  only  one  man  to  do  the  work.  In  using  this  scheme 
the  free  end  of  the  banding  wire  is  soldered  to  the  armature 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     409 

anywhere  and  the  latter  rotated  by  means  of  a  "dog"  and 
pipe  as  indicated  in  the  illustration  until  the  wire  has  filled 
all  of  the  banding  grooves.  In  performing  this  operation  no 
tension  is  exerted  on  the  banding  wire.  Next,  the  remaining 
end  of  the  wire  is  soldered  to  the  armature  and  a  weighted 
pulley  hung  on  a  loop  of  the  banding  wire  nearest  this  end. 

By  selecting  a  weight  twice  as  heavy  in  pounds  as  the  de- 
sired tension  in  pounds  per  square  inch  the  wire  will  be  drawn 
up  to  the  correct  tension  by  rotating  the  armature  backward. 
At  any  time  when  it  is  necessary  to  adjust  the  position  of  the 
banding  wire  this  can  be  done  easily  by  the  person  turning 
the  armature  because  the  tension  will  not  be  relieved  as  would 
happen  when  banding  in  the  ordinary  manner. 


FIG.  259. — Weighted  pulley  in  place  for  producing  tension  in  banding  wire. 

Use  of  a  Crane  to  Band  an  Armature  in  an  Emergency. — 

In  a  large  railway  substation  it  was  necessary  to  replace  the 
banding  wire  on  the  armature  of  a  1000-kw.  rotary  con- 
verter. The  bands  required  about  20  turns  of  steel  wire.  The 
scheme  tried  at  first  was  to  mount  the  steel  wire  reel  so  as  to 
turn-  freely  and  pass  the  wire  through  a  tension  regulator  con- 
sisting of  two  fiber-lined  blocks  bolted  together.  The  wire 
was  then  wound  on  the  armature  as  evenly  as  possible  by 
turning  it  with  bars,  a  method  which  was  found  slow  and 
clumsy.  The  rotary  was  of  the  type  that  is  brought  up  to 
speed  with  an  induction  motor  with  its  rotor  mounted  on  the 
shaft  of  the  rotary  armature.  The  idea  was  therefore  con- 
ceived of  removing  the  stator  of  the  induction  motor  and  using 
the  rotor  as  a  drum  or  pulley  to  rotate  the  rotary  armature. 


410         ARMATURE  WINDING  AND  MOTOR  REPAIR 


Three  turns  of  a  1.25-inch  rope  were  wound  on  this  rotor  and 
one  end  fastened  to  the  hook  of  a  traveling  crane.  The  crane 
was  then  run  the  full  length  of  the  station  with  one  man  feeding 
the  rope  to  the  drum  and  keeping  it  taut.  In  this  way  a 
smooth,  tight  band  was  wound  on  the  rotary  armature  with 
considerable  saving  in  time  over  turning  it  by  hand. 

Method  Used  to  Band  a  2000-Hp.  Rotor. — The  banding  of 
armatures  and  rotors,  when  done  in  a  factory  where  such  jobs 
are  an  every-day  occurrence,  has  been  more  or  less  reduced  to  a 
science  and  the  methods  of  procedure  are  many.  When, 
however,  the  rotor  to  be  banded  reaches  the  2000-hp.  mark, 

Banding 
Wire 


Wire  Tens/on  Device.       Spool. 


-  •  Motor. 

Crane 
Trolley 


'-Flywheel. 


FIG.  260. — An  arrangement  devised  to  band  a  2000-hp.  motor  armature. 

it  is  out  of  the  question  to  think  of  transporting  it  to  the  repair 
shop.  Such  a  problem  came  up  in  a  steel  mill  at  one  time  when 
a  2000-hp.  motor  which  operated  the  rolls  became  seriously 
grounded.  The  following  method  was  decided  upon  (Maurice 
S.  Clement,  Electrical  Record,  April,.  1919) :  The  entire  trolley 
of  a  traveling  crane  was  lowered  to  the  floor  and  placed  in  line 
with  a  huge  fly-wheel  which  was  On  the  same  shaft  as  the  rotor. 
The  cable  drum  of  the  crane  trolley  was  converted  into  a  belt 
drum  by  covering  it  with  canvas  which  was  held  in  place  by 
means  of  friction  tape.  This  made  a  more  or  less  flat  surface 
and  eliminated  both  the  cable  grooves  and  the  grease  from  the 
face  of  the  belt. 

As  the  lower  half  of  the  fly-wheel  was  in  a  pit,  it  was  nee- 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     411 

essary  to  place  a  roller  at  the  edge  of  this  pit  as  a  belt  runner. 
Another  roller  was  placed  on  a  frame  somewhat  resembling 
a  large  brace,  both  ends  of  which  were  secured  to  the  floor, 
halfway  between  the  crane  trolley.  The  fly-wheel  was  ar- 
ranged so  as  to  ride  the  belt  and  serve  as  a  belt  tightener.  The 
banding  wire  tension  device  used  in  this  case  was  similar  in 
principle  to  those  used  on  wire  nail  machines,  the  sheaves 
being  adjustable  in  the  slots  so  as  to  tighten  or  slacken  the  wire 
at  a  moment's  notice. 

An  Improvised  Method  Used  to  Turn  a  Commutator. — 
In  a  case  where  a  rush  order  was  received  by  a  repair  shop 
to  turn  the  commutator  of  a  generator  at  a  large  country 
house,  R.  L.  Hervey  (Electrical  World,  June  10,  1916)  de- 


FIG.  261. — Method  of  using  lathe  cross-feed  in  turning  a  commutator. 

scribes  the  following  method  of  doing  the  work  when  the  small 
turning  rig  which  was  taken  along  was  found  too  small.  In 
the  garage,  however,  a  large  engine  lathe  was  found.  This 
started  an  examination  to  determine  whether  the  armature 
could  be  taken  out  and  swung  in  the  lathe.  This  promised 
impossible.  While  looking  around  the  shop  an  angle  plate 
was  uncovered.  This  plate  and  the  cross-feed  of  the  lathe 
were  bolted  together  by  a  special  screw  of  the  rig.  The  plate 
was  then  bolted  to  the  end  plate  of  the  generator  frame,  as 
shown  in  Fig.  261.  There  was  but  one  bolt  holding  the  angle 
plate  on  the  generator  frame,  which  made  the  equipment  look 
very  unsteady.  A  hole  in  the  angle  plate  was  selected  so  that, 
if  the  plate  turned  around  the  bolt,  the  tool  would  swing 
away  from  the  commutator  and  not  into  it.  The  screw  of  the 


412         ARMATURE  WINDING  AND  MOTOR  REPAIR 

cross-feed  was  too  short  to  take  a  cut  across  the  commutator, 
therefore  the  tool  had  to  be  re-set  for  each  cut.  This  was  a 
little  troublesome  as  the  tool  had  to  be  tapped  into  position 
with  a  hammer.  The  commutator  had  grooves  in  it  about 
%  inch  deep,  requiring  considerable  cutting.  However,  the 
job  was  satisfactorily  done  with  this  arrangement. 

The  generator  was  direct  connected  to  a  low-speed  horizontal 
gasoline  engine.  While  working  on  the  commutator  the  valves 
were  taken  out  to  prevent  compression,  and  the  turning  effort 
provided  by  a  1-hp.  motor  belted  to  the  fly-wheel.  The  elec- 
trical equipment  of  the  plant  consisted  of  the  generator  and  a 
storage  battery  of  58  lead  cells.  These  supplied  current  for 
the  lights,  a  motor-driven  ice  machine,  laundry,  water-pump 
and  machine  tools  in  the  garage. 

Cause  of  Motor  Reversing  its  Direction  of  Rotation  on 
High  Speed. — R.  L.  Hervey  has  described  the  following  pecu- 
liar performance  of  a  motor  (Electrical  World,  Jan.  20,  1917) : 
A  2-hp.  220-volt  direct-connected  compound  motor  driving  a 
band  saw  had  been  in  service  for  approximately  18  months 
without  giving  the  slightest  trouble  until  the  operator 
wanted  to  work  the  saw  at  its  highest  speed,  which  was  ob- 
tained by  using  a  field  rheostat.  When  all  of  the  resistance 
was  cut  in  the  field  circuit  the  motor  slowly  came  to  a  stop  and 
started  in  the  opposite  direction.  This  was  tried  several 
times,  and  resulted  in  blowing  the  fuses  most  of  the  times. 
There  could  be  but  one  condition  that  would  cause  this  re- 
versal of  direction  of  rotation,  namely,  the  compound  winding 
being  in  opposition  to  the  shunt  field  and  overcoming  it  when 
the  shunt  field  was  weakened.  By  reversing  the  compound 
field  leads  the  trouble  was  removed.  This  motor  had  been  in 
operation  for  a  year  and  a  half  without  any  indication  of 
anything  being  wrong.  The  brushes  and  commutator  were 
in  excellent  condition,  which  speaks  well  for  the  design  of  the 
machine. 

Checking  Connections  of  an  Inter  pole  Motor. — The  polarity 
of  the  interpole  field  coils  should  be  the  same  as  that  of  the 
preceding  main  poles  in  a  direction  against  the  rotation.  The 
polarity  of  the  interpole  field  with  relation  to  the  main  fields 
for  both  directions  of  rotation  of  motors  and  generators  is 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     413 

shown  m  Fig.  262.     To  determine  the  proper  connections  of 
interpoles  proceed  as  follows: 

As  a  rule  one  end  of  the  interpole  field  and  one  armature 
lead  are  permanently  connected  together,  and  since  the  correct 


FIG.  262. — Polarity  of  interpole  fields  with  relation  to  main  fields  for  direc- 
tions of  rotation  of  motors  and  generators. 

polarity  is  determined  by  the  manufacturer,  there  is  no  oc- 
casion to  change  it.  To  reverse  the  direction  of  rotation  the 
armature  and  the  interpole  field  must  be  reversed  as  a  unit. 
Reversed  interpole  connection  is  not  noticeable  at  light  load. 
As  load  is  applied,  a  motor  with  reversed  interpoles  will  spark 


Line 


Line 


Series  Ft'efd     *— 7/tf  Field 

Armature  Armature 

FIG.  263, — Connections    for    testing    out    correct    connections    of   interpole 
windings  for  proper  armature  rotation. 

badly  and  drop  in  speed,  while  on  a  generator  difficulty  will  be 
experienced  in  maintaining  rated  voltage.  Assuming  that  one 
end  of  the  interpole  field  and  one  armature  lead  are  perma- 
nently connected,  wire  the  machine  temporarily  as  a  series 
motor  with  the  shunt  field  disconnected.  Care  should  be  taken 


414          ARMATURE  WINDING  AND  MOTOR  REPAIR 

to  apply  the  current  only  momentarily  to  prevent  too  high 
an  armature  speed. 

The  connections  for  this  test  are  shown  in  Fig.  263.  If  direc- 
tion of  rotation  is  wrong,  interchange  terminals  3  and  4  or  1 
and  2.  Having  secured  proper  direction  of  rotation  with  the 
series  connection,  insert  the  shunt  field  and  open  the  series  field 
as  shown  in  Fig.  263.  Upon  starting  up  again  as  a  shunt  motor, 
the  direction  of  rotation  should  be  the  same  as  it  was  with  the 
series  field.  If  it  is  not,  interchange  terminals  5  and  6  and 
make  up  permanent  connections.  The  correct  polarity  of  the 
interpoles  may  then  be  checked  with  a  compass  or  by  bridging 
a  main  pole  and  interpole  with  a  piece  of  iron,  bearing  in  mind 
that  unlike  poles  attract  and  like  poles  will  repel  it  and  check- 
ing with  the  diagrams  in  Fig.  263. 

Heating  of  Field  Coils  Traced  to  Wrong  Type  of  Starting 
Box. — On  motors  that  are  designed  for  field  control  of  speed, 
E.  C.  Parham  has  pointed  out  (Electrical  Record,  February, 
1919)  that  the  shunt  winding  is  likely  to  be  lighter  than  that 
of  a  motor  where  the  shunt  winding  is  to  be  subjected  contin- 
uously to  full  operating  voltage.  Therefore  if  the  shunt  field 
of  such  a  variable  speed  motor  is  connected  to  full  voltage  con- 
tinuously, it  will  heat  beyond  the  usual  temperature  guarantees. 
If  the  line  voltage  happens  to  run  well  above  normal,  as  it  often 
does,  the  temperature  will  reach  a  value  liable  to  injure  the 
insulation  and  ultimately  to  short-circuit  the  winding. 

The  owner  of  a  machine  shop  at  one  time  asked  that  a  man 
be  sent  to  find  out  why  the  speed  of  his  motor  was  so  low  and 
why  the  motor  got  so  hot  after  several  hours  of  working.  As 
this  was  all  the  information  obtainable,  the  inspector  took  a 
voltmeter  and  an  ammeter  that  could  be  used  on  either  direct- 
current  or  alternating-current  circuits,  only  to  find  out  later 
that  no  instruments  were  required.  The  motor  in  question 
proved  to  be  of  the  variable  speed,  field  control,  type.  There 
was  no  name  plate  on  the  motor,  but  the  type  was  inferred  from 
the  type  of  resistance  box  which  had  been  furnished  with  it 
but  which  was  not  being  used.  The  motor  was  being  operated 
as  a  constant  speed  motor  on  voltage  that  was  15  per  cent, 
above  normal.  Of  course  the  maximum  speed  obtainable  was 
that  due  to  full  field  hot.  The  field  coils  heated  too  much  but 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     415 

did  not  heat  sufficiently  for  their  increased  resistance  to  give 
the  speed  required.  In  order  to  bring  the  speed  to  the  nec- 
essary value  and  at  the  same  time  relieve  the  field  coils,  the 
resistance  box,  which  had  many  taps,  was  connected  in  series 
with  the  field  coils  and  different  tap  wires  tried  until  one  was 
found  that  gave  approximately  the  proper  speed.  As  the 
load  of  the  motor  was  comparatively  light  there  were  no  ob- 
jectionable features  attending  the  starting  of  the  motor  on  a 
weakened  field,  so  the  connection  was  made  permanent  with 
the  understanding  that  the  operator  would  immediately  get  a 
starter  that  was  adapted  to  the  work. 

Safe  Operating  Temperature  of  Portable  Desk  Fans.— 
A  well-known  manufacturer  of  fan  motors  points  out  that 
warm  weather  always  brings  a  number  of  complaints  in  re- 
gard to  heating  of  small  motors.  These  complaints  are  usually 
from  dealers  or  users  who  are  possessed  with  the  idea  that  if 
the  motor-body  does  not  feel  nice  and  cool  to  the  hand,  the 
windings  are  in  imminent  danger  of  burning  out.  The  latest 
standardization  rules  of  the  American  Institute  of  Electrical 
Engineers  provide  that,  in  motors  with  the  class  of  insulation 
used  in  reliable  makes  of  motors,  the  temperature  as  recorded 
by  a  thermometer,  should  be  within  a  limit  of  80° C.  This 
is  equivalent  to  176°F.  No  one  would  care  to  place  his 
hand  in  contact  with  the  frame  of  a  motor  at  anywhere  near 
that  limit. 

Let  us  say,  for  instance,  that  a  small  motor  has  a  tempera- 
ture rise  of  40°F.  when  operated  continuously.  Such  a  motor 
operating  on  a  day  when  the  normal  air  temperature  is  70°, 
would  reach  a  temperature  of  only  110°,  which  would  feel  only 
comfortably  warm  to  the  hand.  The  same  motor,  operating 
on  a  mid-summer  day  when  the  thermometer  runs  from  90 
to  95°,  will  attain  a  temperature  between  130  and  140 °F. 
Any  temperature  over  120°  is  quite  uncomfortable  to  the 
touch  and  gives  rise  to  alarm  on  the  part  of  the  inexperienced 
motor  user. 

However,  a  motor  with  a  surface  temperature  of  140° 
is  in  no  danger  of  injury  from  overheating,  and  motor 
users  who  make  complaint  of  the  heating  effect  under  such 
conditions  should  be  advised  to  continue  operating  their 


416         ARMATURE  WINDING  AND  MOTOR  REPAIR 

motors  until  it  is  apparent  that  the  windings  are  being  dam- 
aged. Unless  there  is  some  odor  of  burning  insulation,  the 
motor  can  not  be  considered  in  danger. 

An  Adjustable  Shunt  for  Series  Fields  of  Exciters. — In 
adjusting  the  compound  characteristics  of  three  exciters  for 
parallel  operation  in  conjunction  with  a  Tirrill  regulator,  it 
was  found  impossible  to  get  the  necessary  series-field  shunt 
adjustment  with  the  shunt  taps  provided  by  the  manufac- 


CD 


'  0.010  'German  Silver 


Boned  to  Facilitate  Ventilation 

Toffacn/ne.         To  Conduit 
To^Series Field        terminal^  \ 


Section  cut  from  one 
Strip  to  obtain  Fine 
Adjustment-.. 


Series  Shunt  which  was  Pep  faced 

FIG.  264.- — Construction  of  series-field  shunt  showing  bowed  formation  to 
facilitate  ventilation. 

turers.  Consequently  each  shunt  was  replaced  by  a  very 
simple -and  easily  adjustable  shunt  like  that  shown  in  Fig.  264, 
(Sydney  Fisher,  Electrical  World,  June  10,  1916).  By  means 
of  these  the  characteristics  of  the  machines  were  made  practi- 
cally identical.  The  shunt  was  made  of  strips  of  German 
silver  slotted  to  permit  the  bowed  formation  shown.  Rough 
adjustments  were  made  by  adding  or  removing  strips,  and 
finer  adjustment  obtained  by  varying  the  cross-section  of 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     417 

one  of  the  strips.  The  free  passage  offered  to  air  through  the 
shunt  permits  good  ventilation  with  resulting  low  temperature 
from  no  load  to  full  load.  The  construction  of  the  shunt 
permited  using  the  minimum  number  of  connections,  therefore 
they  can  be  made  much  more  readily  than  where  more  con- 
nections are  required.  This  is  an  important  consideration 
since  the  resistance  of  poor  contacts  becomes  appreciable, 
the  series-field  resistance  being  small. 

A  Peculiar  High-speed  Motor  Trouble. — On  one  occasion 
R.  L.  Hervey  (Electrical  World,  Oct.  28,  1916)  explains  that 
a  500-volt  10-hp.  direct-current  motor,  running  at  a  speed  of 
3400  rpm.,  continued  to  open  its  circuit  breakers  every  time 
an  attempt  was  made  to  start  it.  The  wiring  contractor, 
after  assuring  himself  that  the  connections  were  correct  and 
that  the  trouble  was  inside  the  motor,  refused  to  give  more 
time  to  the  job.  The  local  representative  of  the  machine 
company  claimed  that  every  machine  turned  out  by  his 
factory  was  given  a  24-hour  test,  therefore  the  trouble 
could  not  be  in  the  motor. 

The  motor  was  a  shunt-wound,  bipolar  design,  with  two 
commutating  poles.  The  three  leads  were  marked  A,  F, 
and  C,  which  were  assumed  to  mean  armature,  field  and  line. 
After  testing  out  the  wiring  between  the  starting  box  and  the 
motor,  the  connections  were  made  as  stated  above.  Since 
the  inertia  of  the  machine  parts  to  be  started  was  large  and 
the  speed  quite  high,  a  large  starting  current  was  expected, 
so  that  the  circuit  breakers  were  set  for  their  maximum  current. 
An  effort  was  made  to  start  the  motor  to  test  the  connections 
and  also  to  observe  its  action  when  the  voltage  was  applied,  an 
ammeter  having  been  connected  in  the  circuit  so  that  the 
starting  current  could  be  measured.  When  the  starter 
handle  reached  the  third  button  the  current  was  50  amp. 
The  circuit  breakers  opened  and  the  starter  was  smoking 
badly.  These  results  indicated  an  open-field  circuit.  The 
internal  connections  of  the  motor  were  checked  with  consider- 
able difficulty.  The  lead  marked  A  was  found  to  be  the  line 
and  C  the  armature.  After  making  this  change  the  motor 
was  started  again  with  the  same  results.  The  machine 
representative  objected  to  the  motor  being  taken  apart,  but 

27 


418         ARMATURE  WINDING  AND  MOTOR  REPAIR 

yielded  to  persuasion.  A  small  resistance  tube  connected  in 
the  shunt-field  circuit  was  found  in  the  bottom  of  the  frame. 
While  this  tube  could  not  be  reached  for  examination,  it  was 
suspected  of  being  the  cause  of  the  trouble,  so  a  jumper  was 
connected  around  it.  Another  effort  to  start  the  motor  was 
successful.  The  motor  speed  was  3350  rpm.  with  a  line 
voltage  of  575.  The  resistance,  after  being  patched,  was  con- 
nected in  the  field  circuit  when  the  speed  measured  3730  rpm. 
As  the  motor  was  built  for  3400  rpm.  at  500  volts,  it  was 
thought  that  the  factory  test  was  made  at  that  voltage  and  the 
resistance  put  in  the  field  to  bring  up  the  speed.  Since,  how- 
ever, the  voltage  was  575  at  the  place  of  installation,  the  speed 
of  3350  rpm.  was  high  enough  and  as  the  resistance  had  open- 
circuited  once,  it  was  decided  to  leave  the  resistance  out  to 
prevent  future  trouble. 

Ways  that  End-play  Variations  Show  Up. — The  principal 
purpose  of  having  end-play  in  the  armature  of  a  motor  or  of  a 
generator  is  to  keep  the  brushes  from  tracking  in  the  same 
path  and  thereby  wearing  a  groove  in  the  commutator  or  in  the 
collector  rings  as  the  case  may  be.  To  prevent  such  wearing 
of  grooves,  the  armature  end-thrust  clearances  are  so  disposed 
that  when  the  armature  is  running  in  its  normal  zone,  which  is 
governed  by  the  pull  of  the  pole-pieces  on  the  armature  core, 
the  thrust  clearances  at  the  two  bearings  are  equal.  Assuming 
the  correct  gear  or  pulley  alignment  to  the  connected  load,  the 
armature  will  never  run  sideways  far  enough  to  knock  the 
bearing  on  either  side;  because  when  it  has  run  over  a  certain 
distance,  the  field  will  pull  it  back  toward  and  past  the  mag- 
netic center.  The  same  action  will  then  be  repeated  but  in 
the  opposite  direction.  If,  however,  the  end-thrust  is  un- 
equally distributed  for  any  reason,  or  if  the  machine  as  a 
whole  is  not  level,  knocking  will  occur  which  in  the  case  of 
small,  high-speed  machines  may  take  the  form  of  serious 
vibrations  that  are  likely  to  pound  down  the  bearing 
linings. 

E.  C.  Parham  (General  Electric  Review,  January,  1916)  has 
mentioned  a  case  where  four  motors  of  fractional  horsepower, 
which  were  direct  connected  to  tool  grinding  wheels,  were 
installed  and  three  of  the  outfits  gave  entire  satisfaction,  but 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     419 

the  fourth  one  vibrated  so  badly  that  it  shook  the  whole 
supporting  structure.  The  rotating  elements  of  two  of  the 
units  were  then  interchanged  and  the  vibrations  remained 
with  the  same  rotor.  Spinning  the  rotating  element  of 
the  troublesome  machine  in  a  high-speed  lathe  proved 
the  shaft  to  be  perfectly  straight.  Close  inspection  dis- 
closed that  the  end-play  was  not  distributed  on  the  faulty 
outfit  in  the  same  manner  as  on  the  faultless  ones.  All 
vibration  was  eliminated  by  loosening  the  thrust  collar  so 
that  the  rotor  when  running  could  center  itself,  then  starting 
the  motor  and  letting  it  slow  and  stop,  and  then  tightening 
the  collar. 

Connections  for  Two  220-volt  Motors  When  Operated  on 
440  Volts. — It   is   practical   to   operate   two   220-volt  7-hp. 


Mofor-pu//ey 


fields  are  to  be  separ- 
ate,disconnect  at  X  and 
run  f.  to  field  on  start i, 


''-Driver 


Driven- 


Fields 


Fields 


FIG.  265. — Belt  arrangement  for  two  motors  operating  in  series  and  diagram 
for  connecting  two  shunt  motors  in  series. 


motors  in  series  on  440  volts  and  secure  satisfactory  results, 
providing  that  the  motors  are  approximately  the  same  as 
regards  windings  and  speed.  If  the  drive  is  by  a  belt  and  the 
motors  run  at  the  same  speed  a  good  plan  is  to  set  the  motors 
with  the  pulleys  in  line  and  run  a  short  belt  from  one  to  the 
other.  That  is,  first  belt  the  motors  together  by  a  short  belt 
and  then  put  the  regular  belt  over  the  short  belt  as  shown  in 
Fig.  265.  Another  way  is  to  couple  the  two  armature  shafts 
together  with  a  suitable  coupling  and  attach  the  driver  pulley 
on  one  of  the  armature  shafts. 

The  connections  for  two  shunt-wound  motors  in  series  are 
shown  in  Fig.  265.  The  connections  for  two  compound-wound 
motors  in  series  and  the  connections  for  one  shunt-wound 


420         ARMATURE  WINDING  AND  MOTOR  REPAIR 


motor  and  one  compound-wound  motor  in  series  in  Fig. 
266.  (Frank  Hoskinson,  Electrical  Record,  September,  1917.) 
In  case  one  of  the  motors  is  higher  in  speed  than  the  other 
one,  it  may  be  advisable  to  use  different  sizes  of  pulleys  so  as 
to  secure  the  proper  speed. 


Starting  Box 
Ling   Field  .Arm. 


Starting  Box 

Field\      «^™7- 


Shunt  Fie/cfs 


, 

Shunt  Fields.  Series  Fields 

Shunt  Motor    Compound  Motor 


FIG.  266. — At  the  left,  diagram  for  connecting  two  compound  motors 
in  series  and,  at  the  right,  series  connections  for  one  shunt  motor  and  one 
compound  motor. 

Cleaning  Motors  with  Compressed  Air. — When  using  com- 
pressed air  to  clean  out  dirt  and  dust  from  motor  windings 
the  pressure  should  not  be  higher  than  100  Ibs.  gauge.  The  air 
can  be  applied  by  the  use  of  a  rubber  hose  about  J^  inch  in 
diameter  fitted  with  a  short  piece  of  ^-inch  iron  pipe  to  act 
as  a  nozzle  and  direct  the  air  into  the  windings  so  as  to  blow 
out  accumulated  dirt. 

Most  motor  windings  are  impregnated  or  painted  with  some 
sort  of  insulating  varnish  and  this  usually  presents  such  a 
smooth  surface  that  air,  even  at  extremely  high  velocity,  is 
not  likely  to  lift  the  edges  nor  tear  the  insulating  fabric.  If 
the  dust  that  accumulates  in  the  motor  is  of  an  abrasive  char- 
acter, it  is  by  all  odds  more  advisable  to  use  a  higher  pressure 
than  mentioned  and  get  rid  of  the  dust  than  to  let  the  dust  pile 
up  until  it  stops  ventilation  of  the  windings  and  perhaps  even 
cuts  the  insulation. 

Testing  out  Phase -rotation. — Manufacturers  of  alternating- 
current  generators  generally  have  a  standard  direction  of  rota- 
tion— clockwise,  for  example — and  likewise  there  is  adopted 
a  standard  direction  of  phase-rotation.  This  is  necessary, 
because  alternators  that  are  to  be  operated  in  parallel  must 
have  their  phase-rotations  the  same.  The  adopted  phase- 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     421 

rotation  bears  a  definite  relation  to  the  numbering  of  the 
alternator  terminal  blocks.  For  example,  the  blocks  of  a 
three-phase  alternator  would  be  numbered  1,  2  and  3  and  the 
standard  phase-rotation  would  be  in  the  same  order.  The 
phase-rotation  of  every  alternator  produced  is  tested  by  means 
of  a  phase-rotation  tester.  The  tester  is  virtually  an  induction 
motor — in  fact,  a  standard  motor  could  be  used  as  a  phase- 
rotation  indicator.  The  rotor  of  the  tester,  however,  is  simply 
a  vane  of  iron  free  to  turn  under  the  influence  of  the  stator 
magnetism.  The  stator  winding  of  a  three-phase  indicator 
has  three  terminals,  which  are  marked  1,  2  and  3.  These 
terminals  are  connected  to  corresponding  terminals  of  the 
alternator  the  phase-rotation  of  which  is  to  be  tested.  Re- 
duced voltage  is  applied  to  the  stator  of  the  alternator  and 
the  direction  of  rotation  of  the  vane  of  the  indicator  shows  the 
phase-rotation  of  the  alternator. 

When  an  induction  motor  is  used  as  a  phase-rotation  indica- 
tor it  should  be  connected  first  to  the  supply  end  of  one  ma- 
chine and  then  to  the  other.  If  the  motor  rotates  in  the  same 
direction  it  shows  the  phase-rotation  of  the  two  machines  to 
be  the  same.  If  the  rotation  reverses,  one  phase  of  one  of  the 
alternators  should  be  reversed. 

An  Induction  Motor  Trouble  Due  to  Wrong  Stator  Con- 
nections.— The  importance  of  checking  up  connections  of 
induction  motor  windings  is  shown  by  the  following  trouble 
experienced  by  A.  C.  Hewitt  (Electrical  World,  Jan.  16,  1916). 
In  a  cement  plant  where  a  50-hp.,  three-phase,  440- volt, 
60-cycle  squirrel-cage  induction  motor  operating  at  450 
rpm.  was  used  to  drive  a  33-inch  Fuller  mill  for  pulverizing 
limestone  the  motor  on  several  occasions  was  overloaded 
by  feeding  the  material  to  the  mill  too  fast  and  in  large  sizes. 
This  overloading,  combined  with  fluctuations  of  voltage  as 
much  as  30  per  cent,  below  normal  rating,  caused  the  motor  to 
heat  and  finally  called  for  the  replacing  of  several  stator  coils. 
The  motor  was  repaired  by  the  plant  electrician  and  again 
placed  in  operation,  but  still  it  ran  hot,  with  the  mill  showing  a 
smaller  output  than  a  duplicate  installation  where  the  motor 
was  running  cooler. 

When  called  to  investigate  the  installation  the  repairman 


422         ARMATURE  WINDING  AND  MOTOR  REPAIR 

noticed  that  the  upper  half  of  the  motor  was  much  hotter  than 
the  lower  half.  An  inspection  of  the  connections  seemed  to 
show  that  they  were  correctly  made  and  that  there  were  no 
grounds  or  open  circuits.  The  fuses  were  in  good  condition 
and  the  auto-transformer  motor  starter  operated  satisfactorily. 
All  of  the  connections  for  the  starter  windings  checked  with  the 
diagram  furnished  with  the  motor. 

The  winding  was  a  two-circuit  delta  arrangement  and 
the  motor  had  16  poles.  In  checking  over  the  number  of 
poles  in  each  of  the  two  circuits  it  was  finally  found  that  the 
winding  was  divided  in  a  horizontal  plane  through  the  stator, 
and  that  instead  of  each  circuit  having  eight  poles  the  upper 
circuit  had  only  seven  and  the  lower  had  nine  poles.  This 
explained  why  the  upper  half  ran  hotter  than  the  lower.  The 
electrician  had  made  a  miscount  when  connecting  up  the 
windings,  with  the  result  that  the  load  was  unevenly  distrib- 
uted. The  connections  were  changed  so  that  each  circuit 
contained  an  equal  number  of  poles,  and  the  motor  operated 
without  the  heating  trouble  previously  experienced. 

Stalling  of  Wound  Rotor  Induction  Motor  Explained. — 
A  complete  open  circuit  in  the  wound  rotor  of  a  polyphase 
induction  motor,  will  prevent  its  starting  because  under  such 
conditions  there  can  be  no  secondary  current.  Such  a  com- 
plete open  circuit  may  be  caused  by  two  of  the  rotor  brushes 
being  so  stuck  in  their  holders  as  to  make  no  contact  with  the 
collector  rings.  An  exception  might  obtain  in  the  cases  of 
fractional  horsepower  motors  which  are  so  small  that  the 
rotor  may  start  by  virtue  of  the  eddy  currents  induced  in 
the  rotor  laminations.  However,  the  torque  due  to  such  re- 
actions could  support  no  load.  The  function  of  energy  circuit 
of  a  repulsion  induction  single-phase  motor  is  similar  to  that  of 
the  wound  circuits  of  the  rotor  of  a  polyphase  induction 
motor,  in  that  both  are  the  seat  of  the  induced  current  by 
virtue  of  which  the  motor  is  able  to  do  its  duty  Therefore 
a  complete  break  in  the  energy  circuit  of  a  repulsion  induction 
motor,  will  render  the  motor  unable  to  start.  That  an  in- 
complete open  circuit  in  the  energy  circuit,  may  produce 
different  results,  is  illustrated  by  the  following  experience. 

A  butcher  complained  that  his  motor-driven  meat  grinder 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     423 


which  had  been  working  satisfactorily  for  months,  was  develop- 
ing a  tendency  to  slow  down  when  coarse  stock  was  fed  into 
the  hopper.  With  the  finer  stock,  the  motor  apparently  could 
do  its  work  all  right.  On  operating  the  motor  with  the  end 
cover  removed,  vicious  sparking  was  seen.  At  first  this  was 
thought  to  be  caused  by  armature  trouble  but  a  close  inspec- 
tion disclosed  that  the  sparking  was  due  to  the  energy  brushes 
being  stuck  in  the  brush-holders.  The  brushes  had  worn  so 
short  that  the  brush  shunts  were  jammed  into  the  boxes  and  a 
few  more  jobs  of  grinding  probably  would  have  burned  and 
worn  the  brushes  entirely  out  of  contact  with  the  commutator. 
Even  with  the  arcing,  the  energy  current  had  been  sufficient 
to  support  the  lighter  loads,  although  after  installing  new 
brushes,  it  became  evident  that  the  motor  had  been  operating 
at  reduced  speed  at  all  loads. 

Loose  Bearing  Caused  Induction 
Motor  to  Fail  to  Start.— H.  Wilson 
(Power,  August  5,  1919,  page  231) 
describes  the  following  experience 
after  repairing  a  large  three-phase 
induction  motor,  when  an  attempt 
was  made  to  put  it  back  into 
service.  On  closing  the  compensa- 
tor switch  to  the  starting  position, 
the  motor  failed  to  start,  although 
it  was  evident  from  the  sound  of 
the  machine  that  it  was  getting 
current  through  its  winding.  The 
first  thing  that  suggested  itself  was 
an  open  circuit,  so  the  starting  com- 
pensator was  tested  to  see  if  the 
current  was  coming  through  single- 
phase  only.  In  order  to  do  this 
quickly,  we  disconnected  the  motor 
lead  at  the  machine,  and  while  the 
switch  was  held  on  the  starting 
position  a  test  lamp  was  connected  across  the  different  leads, 
as  in  Fig.  267.  This  showed  current  on  each  phase,  at  a 
reduced  voltage,  of  course.  We  then  put  the  switch  on  the 


3-Phase 

Motor 


FIG.  267. — Method  of  testing 
for  open-circuit. 


424 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


running  position  and  the  test 
lamp  burned  brightly  across 
each  phase,  showing  that  the 
motor  was  getting  the  current 
all  right. 

The  rotor  seemed  to  be  rub- 
bing a  little  on  the  stator,  as 
it  was  somewhat  hard  to  turn, 
so  the  clearance  was  adjusted 
by  means  of  two  draw-bolts 
on  the  bearing  housings.  This 
took  some  time,  since  when  the 
bearings  were  moved  one  way 
a  little,  the  rotor  would  bind; 
then  we  would  shift  it  back 
slightly  until  finally  getting  it 
into  a  position  where  the  rotor 
turned  freely.  Another  at- 
tempt to  start  motor  met  with 
no  better  result  than  the  first. 
The  next  step  was  to  trace  out 
the  winding  connections,  which 
were  found  to  be  apparently 
correct.  The  winding  being 
connected  two  parallel  star 
made  it  somewhat  complicated 
to  trace  out.  However,  by 
looking  to  see  if  the  connec- 
tions went  under  or  over  the 
winding,  I  would  mark  an 
arrow  on  the  group  of  coils  to 
show  the  polarity.  We  went 
around  each  phase  in  this  way, 
starting  with  the  outside  lead 
in  each  case.  The  arrows  on 
each  group  pointed  alternately 
in  opposite  directions,  show- 
ing the  connections  to  be  cor- 
rect, as  in  Fig.  268. 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     425 

The  next  suggestion  was  to  test  the  polarity  of  the  winding 
with  a  compass,  when  direct  current  was  flowing  through  the 
coils.  As  a  source  of  direct  current  an  automobile  starting 
and  lighting  battery  was  pressed  into  service.  The  three 
motor  leads,  A,  B  and  C,  Fig.  268,  were  joined  together  and 
connected  to  one  terminal  of  the  battery.  A  wire  from  the 
other  battery  terminal  was  taken  to  the  common  or  neutral 
point  on  the  winding,  where  the  three  phases  connected 
together,  such  as  A*,  B*  and  C*,  Fig.  268.  This  connection 
was  not  attached  permanently,  as  it  would  have  run  the 
battery  down,  owing  to  the  low  resistance  of  the  circuit  through 
the  winding,  but  was  attached  each  time  only  while  the  swing 
of  the  compass  opposite  each  group  was  obtained.  This  test 
proved  the  connections  to  be  correct,  as  the  compass  needle 
pointed  in  opposite  directions  on  alternate  groups  as  it  was 
moved  around  the  winding. 

Since  the  polarity  of  the  coils  was  undoubtedly  correct,  it 
left  us  all  puzzled  as  to  the  cause  of  the  trouble,  as  we  had 
tested  for  grounds  in  the  winding,  open  circuit,  polarity, 
tested  for  the  power  at  the  motor  terminal,  and  also  mechanic- 
ally, to  see  that  rotor  was  free. 

We  had  about  decided  to  give  the  job  up  and  send  the 
machine  to  the  manufacturers  to  have  it  fixed  up,  when  one 
of  the  engineers  came  in  on  the  job  and  after  hearing  our 
story  remarked  that  the  motor  had  given  similar  trouble, 
two  or  three  years  previous,  after  being  repaired,  due  to  the 
bearing  letting  the  rotor  rub  on  the  stator.  This  gave  us  an 
idea  that  there  might  be  a  slight  looseness  in  the  bearing, 
sufficient  to  let  the  rotor  lift  up  and  rub  on  the  stator.  Acting 
on  this,  the  bearing  bolts  were  adjusted  until  the  rotor  would 
just  clear  the  bottom  of  the  stator.  This  proved  to  be  the 
source  of  all  the  trouble;  on  closing  the  compensator  the  motor 
started  up  and  ran  satisfactorily,  greatly  to  the  relief  of  every- 
body concerned. 

Three-phase  Motors  used  on  Single-phase  Lines. — It 
often  happens  that  utility  customers  have  a  supply  of  three- 
phase  motors  when  only  single-phase  service  is  available,  or 
that  a  customer  is  asked  to  purchase  single-phase  motors  until 
the  load  becomes  large  enough  to  justify  a  three-phase  line 


426 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


extension.  This  difficulty  can  be  overcome  by  a  system  pat- 
ented by  Professor  Arno  in  which  the  inherent  characteristics 
of  three-phase  motors  are  developed  from  a  single-phase  source 
of  supply. 

The  system,  which  is  used  extensively  in  Australia,  calls  for 
the  use  of  a  three-phase  master  motor  in  addition  to  the  three- 
phase  power  motors.  The  master  motor  is  a  standard  machine 
of  either  the  squirrel-cage  or  wound-rotor  type,  and  the  larger 
the  better.  The  lower  size  limit  of  this  motor  is  in  practice 
about  10  to  15  per  cent,  of  the  total  load  connected  to  the  sys- 
tem and  is  at  least  double  the  size  of  the  next  largest  motor. 


MASTZRHOTOR  S^IKRIL  CA6f  HOTOK5        iUPKlNOHOTOK      SOUIRXLCAGtJIcm 

FIG.  269. — Method  of  operating  3-phase  motors  from  single-phase  line. 
The  four  motors  to  the  right  are  power  motors,  and  the  one  to  the  left  is  a  motor 
which  has  its  third  phase  connected  to  the  third  phase  of  all  the  other  motors.  The  mas- 
ter motor  is  started  up  as  a  split-phase  induction  motor  and  runs  light.  Its  function  is 
to  supply  auxiliary  current  to  the  third  phase  of  the  other  motors,  and  by  this  means 
the  3-phase  power  motors  are  given  practically  the  same  characteristics  as  if  they 
were  operated  from  a  3-phase  line. 

All  motors,  including  the  master  motor,  are  connected  to  the 
single-phase  supply  mains,  and  the  third  phases,  which  are 
not  connected  to  the  source  of  supply,  are  connected  together. 
The  master  motor  is  started  up  under  no  load  with  special 
starters  equipped  with  an  auxiliary  winding  similar  to  that 
used  for  the  starting  of  split-phase  induction  motors.  The 
master  motor  should  as  a  general  rule  run  unloaded  as  a  phase 
giver.  It  is  permissible,  however,  to  load  the  master  motor 
up  to  about  25  per  cent,  of  its  normal  rating  in  special  cases, 
but  even  when  the  master  motor  is  running  mechanically 
loaded,  it  may  be  electrically  overloaded.  Any  of  the  power 
motors  becomes  a  master  motor  immediately  its  load  is  thrown 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     427 

off,  and  if  the  master  motor  cannot  properly  perform  its  func- 
tion, a  small  unloaded  power  motor  may  become  overloaded 
in  helping  out  the  master  motor. 

Through  the  voltage  induced  in  the  master  motor's  third 
phase,  which  is  not  connected  to  the  supply  mains,  such  motors 
at  full  speed  are  able  to  supply  auxiliary  current  to  the  loaded 
motors  during  starting  and  overload  periods.  As  long  as  the 
master  motor  is  running  the  overload  capacity  of  the  loaded 
motors  is  raised,  as  compared  with  purely  single-phase  induc- 
tion motors,  and  the  three-phase  motors  are  able  to  start 
up  from  the  single-phase  line  with  practically  the  same  start- 
ing torque  as  they  would  have  when  connected  to  a  polyphase 
line.  Should  all  the  motors  including  the  master  motor  be 
loaded  to  the  same  extent  in  proportion  to  their  rated  capacity, 
no  current  would  flow  in  the  third-phase  connections.  After 
the  master  motor  has  been  started  up,  the  working  motors  can 
be  started  one  after  another  as  regular  standard  three-phase 
motors.  The  gain  in  efficiency  by  operating  the  individual 
motors  as  three-phase  units  is  practically  offset  by  the  losses 
in  the  master  motor,  so  that  the  overload  efficiency  of  the 
whole  installation  is  about  the  same  as  that  of  a  straight  single- 
phase  system.  The  normal  output  of  the  power  motors  when 
operating  in  this  manner  is  about  75  to  80  per  cent,  of  their 
standard  three-phase  rating. 

An  Apparent  Overload  Trouble  That  was  Traced  to  a  Defect- 
ive Fuse  Block. — R.  L.  Hervey  (Electrical  World,  June  24, 
1916)  relates  the  following  trouble  at  a  plating  plant  where  a 
5-hp.  single-phase,  induction  motor,  from  apparent  overload 
continued  to  blow  fuses  for  several  days  and  then  refused  to 
start.  The  motor  had  been  in  daily  use  for  six  years  and  the 
repairman,  after  examining  the  equipment,  said  that  new 
bearings  were  needed  as  the  rotor  was  rubbing  the  stator 
laminations.  The  shaft  was  also  cut  and  grooved,  which 
required  it  to  be  turned  down  so  that  the  bearings  could  be 
properly  fitted.  After  the  motor  had  been  replaced  and  in 
service  for  a  few  days  the  fuses  started  blowing  again. 

The  repairman  found  one  of  the  bearings  hot,  which  was 
taken  out  and  " eased  up"  a  little.  A  few  days  later  another 
shutdown  occurred.  This  time  the  repairman  reported  that 


428         ARMATURE  WINDING  AND  MOTOR  REPAIR 

the  motor  was  overloaded,  and  a  larger  motor  was  required. 
As  the  plant  was  being  operated  exactly  as  it  had  been  for  three 
years  without  a  shutdown,  the  owner  of  the  motor  would  not 
accept  the  report,  and  asked  that  a  thorough  examination 
be  made  to  locate  the  trouble. 

When  putting  in  new  reinforced  fuses  it  was  noticed  that 
the  fuse  clips  and  block  had  been  quite  hot,  for  they  were 
discolored  and  loose.  The  clips  were  closed  up  until  they 
held  the  fuse  tightly.  A  close  examination  of  the  motor  and 
starter  showed  them  to  be  in  first-class  condition.  An  amme- 
ter was  connected  in  the  line  and  the  motor  started  with  no 
load.  The  current  taken  indicated  that  the  winding  was  not 
defective.  The  load  of  two  buffing  wheels  was  then  put  on 
and  the  needle  of  the  50-amp.  meter  went  off  the  scale.  A 
further  examination  '  of  the  fuse  block  showed  two  loose 
connections.  It  was  the  heat  generated  by  this  high  contact 
resistance  that  was  causing  the  trouble  with  the  fuses.  It  is 
very  probable  that  while  the  rotor  was  rubbing  the  stator 
the  motor  was  taking  an  excessive  current  and  injured  the 
fuse  block.  This  instance  shows  the  need  of  a  careful  inspec- 
tion of  all  wiring  when  a  repaired  motor  is  again  connected 
in  circuit. 

Cause  of  Noise  in  a  Three-phase  Motor  Driving  an  Exhaust 
Fan. — The  frequency  of  the  current  that  is  applied  to  the 
stator  of  an  induction  motor  in  commercial  operation,  ordinar- 
ily is  constant  within  narrow  limits  if  the  speed  of  the  prime 
mover  to  which  the  current  is  due,  is  as  constant  as  it  should 
be.  The  frequency  of  the  current  that  is  generated  in  the 
rotor  of  the  motor,  however,  varies  with  the  speed  of  the  rotor, 
being  greatest  when  the  rotor  is  at  rest  and  least  when  the 
rotor  is  operating  at  maximum  speed.  This  is  because  the 
frequency  of  the  rotor  current  depends  on  the  difference  of  the 
speeds  of  the  rotor  and  of  the  rotating  field.  In  other  words 
the  frequency  of  the  rotor  current  depends  on  the  slip  of  the 
rotor.  The  principle  of  operation  of  frequency  indicators  of 
the  vibrating  type  is,  that  the  pendulum  properties  of  each 
reed  are  such  that  it  will  respond  synchronously  to  the  im- 
pulses of  but  one  value  of  frequency.  On  the  same  principle 
if  a  miscellaneous  lot  of  pieces  of  iron  and  steel  be  subjected 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     429 

to  the  flux  of  an  alternating  current  of  varied  frequency,  some 
of  the  pieces  will  take  up  vibration  at  one  value  of  frequency 
and  other  pieces  will  not  be  effected  by  that  value  of  frequency 
but  will  take  up  vibration  at  some  other  value  of  the  frequency. 

In  one  case  a  large  three-phase  induction  motor  that  was 
connected  to  an  exhaust  fan,  was  complained  of  on  account  of 
noise  emitted  and  transmitted  to  all  parts  of  the  building  when 
the  motor  was  being  started.  The  noise  was  described  as  a 
"  clatter. "  An  investigation  disclosed  that  the  noise  occurred 
between  narrow  speed  limits  obtainable  on  the  second  notch 
of  the  controller  and  the  sound  was  similar  to  that  of  rubbing  a 
tin  can  with  a  piece  of  sandpaper.  By  throwing  the  controller 
back  and  forth  between  the  first  and  second  notches,  there 
was  obtained  an  average  speed  at  which  the  noise  was  main- 
tained almost  continuously.  By  listening  with  the  ear  applied 
to  different  parts  of  the  machine,  and  at  the  same  time  touch- 
ing the  machine  here  and  there  with  a  lead  pencil,  the  source  of 
the  trouble  was  located  in  one  of  the  slots  in  which  slide  the 
bolts  that  hold  the  motor  frame  to  the  sliding  base.  On  each 
side  of  the  machine  extending  from  one  bolt  to  the  other,  a 
steel  strip  was  provided  in  order  to  keep  the  head  of  the  bolt 
from  turning  when  there  was  occasion  to  turn  its  nut  with  a 
wrench.  The  strip  was  so  " bellied"  that  there  was  about  J^g 
inch  clearance  between  the  middle  of  the  strip  and  the  bottom 
of  the  machine  which  effectively  constituted  a  reed  supported 
at  both  ends,  and  the  time  element  of  the  construction  was 
such  that  the  strip  vibrated  in  response  to  rotor  current  fre- 
quency that  obtained  on  the  second  notch  of  the  controller. 
On  removing  the  strip  and  re-bending  so  as  to  bring  the  bulged 
part  up  against  the  bottom  of  the  rail,  no  further  trouble  was 
experienced  with  noise. 

Cause  of  a  Burned  Out  Starting  Winding  in  a  Single-phase 
Motor. — Unless  of  the  commutator  type,  a  single-phase 
motor,  in  order  that  it  may  be  started  without  manual  assist- 
ance, must  include  some  phase-splitting  device  that  is  effective 
in  producing  the  rotating  field  so  necessary  to  self -starting 
of  such  motors.  The  phase-splitting  device  usually  takes  the 
form  of  an  auxiliary  starting  winding  and  of  an  arrangement 
of  resistance  and  inductance  or  of  resistance  and  reactance, 


430         ARMATURE  WINDING  AND  MOTOR  REPAIR 

so  disposed  that  the  currents  in  the  main  stator  winding  and 
in  the  starting  winding  shall  be  out  of  phase  with  each  other. 
As  the  starting  winding  is  intended  to  be  active  for  only  a 
second  or  more,  it  is  not  proportioned  to  stand  continuous 
application  of  the  voltage.  Accordingly,  a  centrifugal  switch 
that  turns  with  the  rotor,  is  provided  for  automatically  cutting 
out  the  starting  winding  as  soon  as  the  rotor  approaches  full 
speed.  If  for  any  reason  the  starting  winding  is  left  in  circuit 
for  appreciable  time,  the  winding  is  likely  to  be  injured. 

In  a  certain  instance  (Electrical  Record,  August,  1918)  one 
of  these  motors  stopped  a  few  minutes  after  having  been 
started  for  the  first  time  and,  as  the  owner  expressed  it,  "the 
motor  had  no  more  life  in  it."  The  motor  was  of  the  resistance- 
inductance  self -starting  type,  in  which  the  phase  splitting  was 
accomplished  without  the  use  of  any  external  resistance  or 
reactance.  Examination  of  the  motor  disclosed  that  its 
failure  had  been  due  to  the  main  stator  winding  being  in 
series  with  the  automatic  switch  which  should  have  included 
only  the  starting  winding.  The  motor  would  start  all  right, 
but  the  opening  of  the  centrifugal  switch  would  cut  out  the 
main  winding,  leaving  the  starting  winding  connected  across 
the  line.  It  developed  that  the  motor  had  been  bought  at  a 
low  price  because  it  was  in  bad  condition  and  had  been  re- 
wound by  a  local  repair  man,  who  did  o  good  job  of  winding, 
but  a  poor  job  of  connecting. 

Cause  of  One  Motor  Failing  to  Start  While  Another  was 
Running  on  the  Same  Circuit. — At  an  industrial  plant  where 
440- volt,  three-phase,  60-cycle  service  was  used  to  operate 
motors  of  many  different  sizes,  a  peculiar  trouble  was  experi- 
enced by  A.  C.  Hewitt  (Electrical  World,  Jan.  8,  1916)  when 
one  of  three  fuses  was  blown  on  a  feeder  circuit.  One  feeder 
circuit  supplied  some  four  or  five  motors,  one  of  which  was 
rated  at  75  hp.  and  the  others  at  10  hp.  each.  Each  motor 
was  equipped  with  a  knife  switch,  fuses  and  starting  compensa- 
tor, and  the  feeder  circuit  was  fused  at  the  distributing  switch- 
board. The  75-hp.  motor  was  delta-connected,  while  the 
10-hp.  motors  were  connected  "  Y"  (Fig.  270). 

The  75-hp.  motor  was  carrying  a  load  of  about  50  hp.  Only 
one  of  the  10-hp.  motors  was  in  use,  and  it  was  driving  a  7-hp. 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     431 


load  at  the  time  the  trouble  developed.  Both  motors  had 
been  running  for  about  four  hours  when  it  became  necessary 
to  stop  the  small  motor  for  a  few  minutes.  Upon  trying  to 
operate  it  again  it  refused  to  start.  The  electrician  tested 
the  motor  fuses  with  two  250-volt  lamps  in  series,  without 
removing  the  fuses  from  the  circuit,  and  found  them  good. 
The  lamp  test  seemed  to  show,  however,  that  the  voltage  be- 
tween the  middle  and  either  outside  wire  was  not  as  high 
as  it  should  be.  He  then  tested  the  fuses  on  the  75-hp.  motor 
while  the  motor  was  running  and  found  tho  same  conditions 


r Leads  to  Other  Motors--* 


Fuses 


Starting 
Compensotor 


lOHp 
Motor 


75  Hp  Motor 
FIG.  270. — Connections  when  10-hp.  motor  failed  to  start. 

as  with  the  10-hp.  motor.  Evidently  something  was  wrong, 
and  with  reduced  voltage  between  the  middle  and  outside 
wires  it  was  natural  to  suppose  that  there  was  a  high-resistance 
connection  somewhere  in  that  feeder.  The  large  motor  was 
stopped,  and  the  fuses  were  tested  as  before,  but  this  time  no 
voltage  at  all  was  found  between  the  middle  and  the  outside 
wires.  Then  the  electrician  tried  to  check  up  the  first  results 
by  starting  the  large  motor,  but  since  it  was  a  squirrel-cage 
three-phase  induction  motor  it  would  not  start  on  a  single- 
phase  circuit. 

He  next  tested  the  main-feeder  fuses  and  found  one  of  them 
had  blown.  A  new  feeder  fuse  was  put  in,  and  everything 
started  off  all  right.  The  reduced  voltage  between  the  middle 
and  the  outside  wires  was  just  one-half  of  the  line  voltage, 
or  220  volts,  since  the  middle  wire  amounted  to  a  50  per  cent. 


432 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


tap  on  the  winding  of  the  large  motor  when  it  was  running 
single-phase.  This  case  seemed  quite  a  puzzler  at  the  time, 
and  it  was  thought  at  first  that  the  reduced  voltage  on  the 
middle  wire  was  being  generated  in  the  large  motor.  How- 
ever, a  little  study  of  the  conditions  soon  made  plain  the  real 
reason  as  explained. 

Cause  of  Synchronous  Motor  Failing  to  Start. — E.  C. 
Parham  (Power,  June  24,  1919)  explains  the  following  trouble 
with  a  synchronous  motor  in  a  large  mill  when  it  was  necessary 
to  repair  the  winding  which  had  been  damaged  in  a  fire. 


FIG.  271. — Pole-phase  group  shown  reversed  at  X. 

A  'complete  set  of  coils  was  available  and  the  rewinding  was 
done  by  local  repairmen.  After  completing  the  job,  however, 
connecting  the  motor  to  its  source  of  power  failed  to  turn 
the  rotor,  although  it  had  started  very  promptly  before  being 
damaged.  Higher  compensator  taps  were  tried,  but  to  no 
avail.  An  expert  alternator  winder  was  called  from  the 
factory  and  found  that  one  pole-phase  group  of  coils  had  been 
reversed  as  at  X,  Fig.  271.  The  interchanging  of  the  leads 
of  this  group,  as  in  Fig.  272,  restored  normal  starting  torque. 
It  will  be  noticed  that  by  tracing  each  phase  through  to  the 
star  connection  in  Fig.  272,  the  arrows  point  in  opposite  direc- 
tions on  adjacent  pole-phase  groups,  which  is  the  correct 
condition.  In  Fig.  271  the  wrong  connection  is  indicated 
by  three  adjacent  arrows  pointing  in  the  same  direction. 

The  existence  of  such  a  condition  may  be  proved  by  con- 
necting ammeters  into  each  phase  conductor  and  observing 
the  intake  currents  while  the  rotor  is  turned  by  hand;  a  re- 


METHODS  USBD  TO  SOLVE  SPECIAL  TROUBLES     433 

versed  pole-phase  group  of  coils  will  greatly  unbalance  the 
currents  of  the  three  wires.  It  is  essential  that  the  rotor  be 
rotated  so  as  to  equalize  impedances  due  to  different  phases 
including  different  amounts  of  iron  in  their  magnetic  circuits. 


FIG.  272. — Correct  connection  for  a  4-pole  series-star  winding. 

Effect  of  Decreased  Frequency  on  Operation  of  an  Induc- 
tion Motor -generator  Set. — The  immediate  effect  of  operating 
a  generator  below  its  rated  speed  is  to  increase  the  amount  of 
field  current  required  in  order  to  maintain  normal  voltage. 
This  is  due  mainly  to  the  fact  that  the  maintenance  of  normal 
voltage  under  given  load  conditions  requires  that  certain 
number  of  field  lines  of  force  be  cut  each  second  by  the  arma- 
ture conductors,  and  if  the  necessary  rate  of  cutting  can  not 
be  obtained  from  the  existing  conductor  rate  of  motion  through 
the  existing  field,  the  field  strength  must  be  increased  until 
the  increase  makes  up  for  the  lack  of  conductor  speed.  If 
the  field  strength  can  not  be  increased  in  proportion  to  the 
speed  deficiency,  voltage  can  not  be  maintained  at  its  normal 
value. 

One  effect  of  decreasing  the  frequency  of  the  voltage  that  is 
applied  to  an  induction  motor  is  to  decrease  proportionally 
the  speed  of  the  motor  and  of  its  connected  load.  It  follows, 
then,  that  if  an  induction  motor  is  the  driving  member  of  a 
motor-generator  set  of  which  the  driven  member  is  a  con- 
tinuous-current generator,  the  effect  of  low  frequency  will  be 
to  reduce  the  speed  of  the  set.  If  the  speed  decrease  is  too 
great,  the  generator  field  will  not  have  sufficient  margin  to 
permit  of  maintaining  normal  voltage  at  full  load — the  load 

28 


434          ARMATURE  WINDING  AND  MOTOR  REPAIR 

itself  producing  further  speed  reduction  incident  to  the  in- 
crease of  the  amount  of  motor  slip. 

E.  C.  Parham  (Electrical  World,  Jan.  1,  1916)  has  referred  to 
such  a  situation  where  an  operator  on  a  50-cycle  system 
specified,  in  ordering  a  small  motor-generator  set,  a  60- 
cycle  motor  because  he  knew  that  ultimately  his  service  would 
require  60-cycle  operation.  In  ordering  the  60-cycle  set 
for  temporary  50-cycle  operation  he  had  considered  only  the 
motor,  and  had  felt  safe  in  assuming  that  it  could  stand  the 
larger  current  incident  to  operating  on  the  lower  frequency. 
The  safety  of  such  an  assumption  would  largely  depend  on  how 
nearly  the  motor  would  operate  at  its  normal  rating  under  the 
prospective  load  conditions  and  with  the  proper  frequency. 
The  point  of  interest  in  the  present  instance,  however,  is  that 
when  the  60-cycle  set  was  put  into  service  it  was  found 
impossible  to  maintain  the  generator  voltage  at  normal  value 
even  with  the  generator  field  rheostat  resistance  all  cut  out. 
In  order  to  get  out  of  the  difficulty  it  was  necessary  to  install 
upon  the  generator  a  set  of  shunt-field  coils  that  had  a  greater 
number  of  ampere-turns. 

Alternating-current  motors  are  designed  and  are  rated  to 
stand  a  reasonable  departure  from  their  voltage,  current  and 
frequency  specifications,  but  before  abusing  them  with  eyes 
wide  open  it  would  be  advisable  for  operators  to  ascertain  the 
permissible  limits  of  abnormal  use. 

Simple  Rules  for  Re-connecting  Alternating-current  Motors. 
— The  repairman  should  devise  simple  rules  that  he  can  re- 
member easily  so  as  to  be  able  to  handle  simple  changes  in 
motors  without  having  to  study  up  the  connections  or  ask  his 
neighbor  to  explain  them.  A  few  such  suggestions  are  given 
here  as  used  by  Maurice  S.  Clement  (Electrical  Record,  October, 
1918)  for  re-connecting  a  motor  to  suit  changes  in  voltage, 
speed,  etc.  For  example,  if  a  three-phase,  four-pole  motor 
is  to  be  changed  to  two-phase,  four-pole,  at  the  same  time 
retaining  the  same  speed,  the  grouping  must  be  changed. 
A  three-phase,  four-pole  connection  has  12  groups;  that 
is,  one  group  to  each  pole,  of  which  there  are  four,  and 
four  poles  to  each  phase,  giving  12  groups.  Therefore,  the 
numDer  of  poles  multiplied  by  the  number  of  phases  givos 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     435 

the  number  of  groups.  By  this  rule  a  two-phase,  four- 
pole  machine  has  eight  groups.  The  next  element  to  be 
taken  into  consideration  is  the  number  of  coils  in  each 
group. 

In  a  three-phase,  four-pole  machine  with  48  coils  there 
would  be  four  coils  per  group;  with  36  slots,  three  coils  per 
group.  In  a  48-coil,  four-pole,  three-phase  machine  there  are 
12  groups  of  four  coils  each.  To  change  this  to  a  two-phas?, 
four-pole  winding  without  change  of  speed,  the  following  is 
the  rule: 

(Number  of  coils)  +  (Number  of  poles  X  Phases) 

=  48  -T-  (4  X  2)  -  6. 

Therefore  the  new  grouping  will  be  arranged  with  six  coils 
per  group. 

In  changing  the  grouping  of  a  36-coil  machine  from  four- 
pole,  three-phase  to  four-pole,  two-phase,  the  result  becomes 
a  trifle  puzzling,  as  it  apparently  gives  four  and  one-half 
coils  per  group.  Since  we  know  that  cutting  a  coil  in  half 
to  suit  a  particular  grouping  can  not  be  done,  the  grouping  of 
coils  will  be  in  the  following  order:  4,  4,  5,  5,  4,  4,  5,  5.  This 
method  accounts  for  the  half  coils  and  will  be  found  to  be 
evenly  balanced.  If  the  machine  is  to  be  changed  from  two- 
phase  to  three-phase,  simply  reverse  the  operation  that  is 
described  above. 

For  general  details  for  connecting  alternating  current  motors, 
see  Chapter  XI. 

Changing  440-volt  Motor  for  220-volt  Operation. — The 
accompanying  illustrations  show  how  a  440-volt,  4-pole, 
2-phase  motor  was  reconnected  for  220-volt  operation  by 
reconnecting  the  coils  in  parallel.  The  diagram  of  Fig.  273  (a) 
shows  the  original  connections  of  the  motor,  and  Fig.  273  (6) 
indicates  the  connection  after  the  necessary  alterations  had 
been  made.  The  changes  indicated  in  the  diagram,  Fig. 
273  (6),  were  made  as  follows: 

Terminal  Xi  was  joined  to  the  beginning  of  coil  group 
A i  and  end  of  group  A4.  Then  the  beginnings  of  groups  As 
and  A 4  and  ends  of  A\  and  A2  were  connected,  Terminal  Xz 


436 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


was  joined  to  the  beginning  and  end  of  groups  A 2  and  As 
respectively.  The  5-phase  coils  were  connected  in  a  similar 
manner. 

The  foregoing  alterations  give  very  well-distributed  end 
windings,  which  are  considered  essential  to  obtain  a  uniform 
flux  distribution;  but  the  impedance  of  the  parallel  circuits 
will  be  changed  if  the  rotor  runs  off  center,  as  may  happen 
with  a  worm  bearing,  thus  causing  overheating.  On  account 
of  the  small  air  gap  usually  found  in  alternating-current 
motors,  there  is  small  probability  of  this  happening  because 
worn  bearings  would  cause  the  rotor  to  rub  the  stator  before 


v, 


x, 


FIG.  273. — Motor  winding  before  and  after  changing  from  440  volts  to  220 

volts. 


it  would  bring  about  overheating.  Unequal  distribution 
of  current  between  parallel  windings  due  to  a  change  of  imped- 
ance may  be  avoided  by  connecting  the  coil  groups  which  are 
diametrically  opposite  Of  course,  this  cannot  be  done  with 
two  parallel  windings  if  one-half  the  number  of  poles  is  an 
odd  number. 

Multiple  Connection  Diagram  for  A.-C.  Motor  Windings.  — 
Maurice  S.  Clement  of  Youngstown,  Ohio,  makes  use  of  the 
diagram  shown  in  Fig.  274  and  claims  that  it  saves  a  great  deal 
of  time  in  re-connecting  alternating-current  motors  (Electrical 
Record,  February,  1919).  This  multiple  connection  winding 
diagram  takes  the  place  of  four  diagrams,  since  it  can  be  used 
to  show  connections  for  single  star,  single  delta,  parallel  star 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     437 


and  parallel  delta   windings  for   alternating-current  motors. 

In  this  instance  a  four-pole,  three-phase  diagram  is  shown,  but 

the  same  plan  can  be  used  with  different  polarity  and  phasing. 

The  following  are  the  formulas  for  the  different  connections : 


SINGLE   STAR 


7,  9,  11  =  Line 

8  &  10  &  12  =  Star 

1  &  4     2  &  5     3&6in  Series 


PARALLEL    STAR 

4  &  7  =  Line 
6  &  9  =  Line 
2  &  11  =  Line 
1&3&5  8&10&12=  Stars. 


SINGLE  DELTA 

7  &    8  =  Line 

9  &  10  =  Line 

11  &  12  =  Line 

1&4  2&5     3&6in  Series 

PARALLEL   DELTA 

7&  4  &  8  &  5=  Line 
2  &  11  &  3  &  12  =  Line 
1&10&9&  6=  Line 


When  connecting  a  stator  it  is  always  well  to  look  forward 
to  the  fact  that  some  day  the  connections  may  be  changed  to 
suit  other  conditions,  and  that  if  the  long  connection  is  used 
it  will  be  much  easier  to  change.  By  long  connection  is  meant 
this.  If  the  reader  will  trace  out  the  first  phase  of  a  single 
star  on  the  sketch,  he  will 
find  it  to  be  1  to  7  to  10  to 
4;  whereas,  it  is  sometimes 
connected  as  follows:  1  to 
4  to  7  to  10.  By  study- 
ing the  sketch  closely  it  will 
be  seen  that  if  the  long 
connection  method  is  used, 
the  connection  can  be 
changed  to  any  of  the  other 
three  much  easier  than  by 
the  latter  method,  as  half 
the  connections  do  not  have 
to  be  opened.  Although 
the  short  connection  is  per- 
haps a  wire  saver  by  a  couple  of  inches,  the  long  connection 
is  easier  to  understand  and  makes  a  much  neater  job. 

Brush  and  Slip-ring  Sparking  Traced,  to  Absence  of 
Rotor  Balancing  Weights. — Manufacturing  companies  take 
particular  care  that  the  rotors  of  alternating-current  machines 
and  the  armatures  of  direct-current  machines  are  perfectly 


FIG.  274. — Diagram  that  can  be  used 
for  connecting  terminals  of  pole-phase- 
groups  for  single  and  parallel  star  and 
delta  connections. 


438         ARMATURE  WINDING  AND  MOTOR  REPAIR 

balanced  before  they  are  shipped  either  .in  machines  or  as 
extras.  Some  go  so  far  as  to  balance  rotors  before  and  after 
installing  the  coils  and  the  higher  speed  rotors  of  the  slip-ring 
type  may  be  balanced  before  and  after  installing  the  slip  rings. 
Most  of  the  balancing  weight  is  applied  by  forcing  lead  into 
pockets  provided  in  the  core  end  plates  for  that  purpose. 
That  the  absence  of  these  weights  may  set  up  vibrations  that 
cause  roughening  of  the  slip  rings  and  ultimate  sparking,  is 
proven  by  the  following  experience: 

In  one  shop  (Electrical  Record,  August,  1918)  a  slip-ring  motor 
had  given  much  brush  and  slip-ring  trouble.  Turning  of  the 
rings  and  changing  of  the  rings,  brushes  and  holders,  had  given 
no  permanent  relief.  A  factory  inspector  at  once  diagnosed 
the  cause  of  the  trouble  as  vibration — which  no  one  had  sus- 
pected because  it  was  in  one  of  those  shops  where  everything 
that  moves  vibrates.  He  tested  the  rotor  in  a  lathe  and  found 
that  the  shaft  was  bent.  The  shaft  was  then  straightened 
and  it  was  thought  that  all  trouble  was  at  an  end;  but  not  so. 
The  machine  ran  better,  but  there  was  considerable  vibration 
still  there. 

On  questioning  the  operator,  the  statement  was  obtained 
that  the  motor  had  run  all  right  up  to  the  time  that  it  had 
passed  through  a  fire  about  a  year  previously.  The  fire  had 
melted  out  the  bearings  and  had  destroyed  everything  in  the 
nature  of  insulation;  also  it  had  distorted  the  shaft  and  melted 
the  balance  weights  although  no  one  had  considered  the  pos- 
sibility of  the  last-named  conditions  at  the  time  of  re-winding 
che  machine  and  installing  new  bearings.  The  rotor  was  sent 
back  to  the  factory  with  a  statement  of  its  history  and  when 
it  was  returned  and  re-installed  the  motor,  in  the  words  of  the 
owner,  "ran  like  a  new  machine. "  Probably  it  was  a  new 
machine, 

Overheating  of  an  Induction  Motor  Traced  to  a  Variation  of 
Frequency. — Provided  that  an  induction  motor  is  not  already 
overloaded,  it  is  not  considered  abusive  to  operate  the  motor 
at  less  than  10  per  cent,  over  voltage  or  at  10  per  cent,  under 
frequency.  Manufacturers  are  careful  to  specify,  however, 
that  the  motor  should  not  be  expected  to  stand  without  mate- 
rial increase  in  heating,  the  continuous  application  of  10  per 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     439 

cent,  over  voltage  and  10  per  cent,  under  frequency,  because 
both  of  these  variations  are  in  the  direction  that  tends  to  in- 
crease the  heating.  If  the  ammeter  of  an  induction  motor 
circuit  be  observed  while  the  motor  is  being  started,  it  will 
be  seen  that  when  the  motor  reaches  nearly  normal  speed,  the 
current  drops  suddenly  to  a  value  far  below  what  existed  imme- 
diately preceding  the  sudden  drop.  It  is  conceivable  that 
certain  voltage-frequency-load  conditions  might  exist  whereby 
the  motor  would  be  operating  just  on  the  high  side  of  the  crit- 
ical point  indicated.  Under  this  condition  the  current  would 
be  abnormally  large. 

In  one  case  a  mill  operator  complained  of  the  heating  of  one 
of  his  larger  induction  motors.  He  admitted  that  the  motor 
was  overloaded,  but  the  overload  was  constant  and  this  did 
not  explain  why  the  heating  was  so  much  greater  during  some 
periods  than  during  others.  When  taking  the  motor  speed 
during  a  period  of  maximum  load,  the  inspector  was  much 
surprised  to  find  that  the  speed  was  five  per  cent,  above  syn- 
chronism. The  mill  operator  stated  that  over  speeds  had  never 
been  noticed,  but  he  felt  certain  that  there  were  times  when 
the  speed  was  considerably  under  normal.  Investigation  of  the 
engine  room  equipment,  disclosed  that  at  times  the  steam  pres- 
sure became  so  low  that  it  was  necessary  for  the  engineer  to 
operate  the  overload  valve  of  the  turbine  in  order  to  maintain 
its  speed,  hence  frequency.  The  turbine  generator  was  excited 
from  an  engine-driven  exciter,  the  speed  of  which  was  not 
materially  affected  by  the  variations  of  steam  pressure.  As 
the  frequency  of  the  service  applied  to  the  motors  depended 
on  the  speed  of  the  turbine,  which  speed  depended  on  steam 
conditions,  while  the  voltage  ,of  the  service  applied  to  the 
motors  depended  on  the  turbine  voltage  which  in  turn  de- 
pended not  only  on  the  turbine  speed  but  on  the  turbine  excita- 
tion which  was  independent  of  steam  pressure  variations,  the 
extreme  conditions  of  high  voltage  and  low  frequency,  which 
may  have  obtained  at  times,  can  better  be  imagined  than 
estimated.  As  the  engine  room  conditions  could  not  imme- 
diately be  relieved,  the  duty  of  the  abused  motor  was  lightened 
by  removing  a  few  of  the  connected  machines  that  constituted 
its  load. 


440         ARMATURE  WINDING  AND  MOTOR  REPAIR 

Relief  for  a  Hot  Bearing. — In  case  of  a  hot  bearing,  feed 
plenty  of  heavy  oil,  loosen  the  nuts  on  the  bearing  cap  and 
slacken  the  belt  if  one  is  used.  If  no  relief  is  shown,  take  off 
the  load  and  run  the  machine  slowly  until  the  shaft  is  cool 
so  that  the  bearing  will  not  "  freeze. "  If  the  bearing  is  of 
babbitt  examine  it  to  see  that  the  oil  grooves  are  still  intact. 
If  they  are  and  the  surface  of  the  bearing  has  not  been  injured, 
renew  the  oil  supply  and  start  the  machine  again.  Watch 
the  oil  rings  to  see  that  they  are  revolving  properly  and  carry- 
ing plenty  of  oil  to  the  shaft.  A  new  machine  or  one  in  which 
the  bearings  have  been  renewed  should  be  run  at  slow  speed 
for  an  hour  or  more  before  the  load  is  applied  in  order  to  see 
that  the  bearings  are  properly  adjusted  and  worked  in. 

Static  Sparks  from  Belts  (Instruction  Book,  Westinghouse 
Electic  &  Mfg.  Co.). — It  sometimes  occurs  on  belted  machines, 
especially  in  dry  weather,  that  charges  of  static  electricity 
of  considerable  potential  on  the  belt  cause  discharges  to  the 
ground.  If  the  frame  of  the  machine  is  not  grounded,  these 
charges  may  jump  to  the  armature  or  field  winding  and  then 
to  the  ground,  puncturing  the  insulation.  The  belt  and  frame 
may  be  discharged  by  placing  close  to  the  belt,  at  a  point  near 
the  machine  pulley,  a  number  of  sharp  metal  points  like  a 
comb,  which  are  carefully  grounded.  If  the  field  frame  is 
grounded,  there  should  be  no  danger  to  the  insulation. 

Ratings  of  A.-C.  Generators. — In  the  case  of  a  single-phase 
generator,  the  rating  in  kva.  is  equal  to  the  product  of  amperes 
and  volts. 

For  a  two-phase  generator,  the  rating  in  kva.  is  equal  to 
twice  the  output  of  one  phase  when  the  load  is  balanced. 

For  a  three-phase  generator  the  total  rating  in  kva.  is  equal 
to  the  output  of  one  phase  multiplied  by  1.732.  That  is,  the 
readings  or  amperes  and  volts  for  one  phase  times  1.732  is 
equal  to  the  kva.  rating  of  the  machine,  when  it  carries  a 
balanced  load. 

Alternating-current  Motor  Phase-rotation  (Instruction  Book, 
Westinghouse  Electric  &  Mfg.  Co.). — In  order  that  the 
alternating-current  wiring  connections  between  the  motor 
and  its  supply  circuit  may  be  correctly  made  to  obtain  a  given 
direction  of  rotation,  it  is  necessary  to  know  the  phase-rota- 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     441 

tion  of  the  motor  and  the  supply  circuit.  By  "  phase-rotation 
of  the  motor"  is  meant  the  order  in  which  each  phase  reaches 
its  maximum  voltage  of  one  polarity.  When  the  machines 
are  arranged  for  clockwise  rotation,  looking  at  the  commutator 
end,  the  phase-rotation  is  given  by  the  order  of  terminal 
notation.  Using  letters  to  designate  the  terminals  of  West- 
inghouse  machines,  the  order  for  three-phase  machines  is 
B-C-A  and  for  two-phase  Bi-Ai  B2-A2. 

The  sequence  of  phase-rotation  of  the  supply  system  can  be 
found  by  tracing  the  wiring  back  to  the  generating  station  or 
else  by  the  use  of  a  phase-rotation  indicator.  When  there  are 
flexible  cable  leads  on  the  alternating-current  side,  the  simplest 
method  of  determining  the  proper  order  in  which  to  connect  the 
motor  leads  is  to  connect  the  leads  in  any  convenient  order  and 
start  the  motor.  If  the  direction  of  rotation  is  opposite  to 
that  desired,  reverse  any  two  leads  in  the  case  of  a  three-phase 
motor  or  interchange  the  two  leads  of  either  phase,  in  the  case 
of  a  two-phase  motor. 

End  Bells  or  Heads. — An  end  bell  which  is  cracked  can 
easily  be  welded,  which  process  makes  it  just  as  good  as  new. 
If  a  bearing  has  been  worn  out  or  burned  out,  it  must  be  re- 
lined.  This  work  must  be  done  by  a  machinist  who  appre- 
ciates the  importance  of  a  good  bearing. 

Brushes  and  Brush  Holders. — When  a  brush  holder  is 
damaged  to  any  great  extent,  it  is  advisable  «to  order  a  new 
set  from  the  factory  as  it  would  not  pay  to  make  a  set  in  a 
repair  shop.  Carbon  brushes  are  easily  made,  however,  and 
dimensions  should  be  exact  so  as  to  insure  their  working  easily 
in  the  holder. 

The  Rotor. — Under  certain  conditions,  a  rotor  will  become 
so  heated  as  to  cause  the  solder  to  melt  from  its  bars.  When 
this  happens  the  affected  parts  will  rattle  when  the  machine 
is  run;  resoldering  the  bars  will  remedy  this.  If  the  bars 
become  damaged  or  bent,  they  should  be  removed,  straight- 
ened, the  slots  reinsulated  and  rotor  reassembled. 

The  Stator. — The  troubles  occurring  in  an  alternating-cur- 
rent stator  may  be  said  to  be  very  similar  to  those  of  an  arma- 
ture and  most  of  the  tests  described  can  be  used.  For  details 
of  checking  up  errors  in  connections  see  Chapter  XI,  page  288. 


442          ARMATURE  WINDING  AND  MOTOR  REPAIR 


Sizes  of  Fuses  for  A.-C.  Motors. — The  National  Electrical 
Code  does  not  specify  the  size  of  wire  which  should  be  used  to 
connect  up  any  given  motor,  nor  does  it  give  the  sizes  of 
starting  and  running  fuses  to  be  used.  A  committee  of  the 

SQUIRREL-CAGE    THREE-PHASE    INDUCTION    MOTORS    EQUIPPED    WITH 
AUTO-STARTERS 


Average 
horsepower 

Full-load  amp. 

Starting  fuse  amp. 

Running  fuse  amp. 

220  volts 

550  volts 

220  volts 

550  volts 

220  volts 

550  volts 

0.5  
1.0  

1.8 
3.5 
6.5 
9.5 
15.4 
22.4 
29.0 
42.5 
55.0 
68.0 
80.0 
94.0 
105.0 
130.0 
155.0 
192.0 
252.0 
368.0 
484.0 
595.0 
710  0 

0.7 

1.3 
2.6 
3.8 
6.2 
9.0 
11.8 
17.4 
22.5 
27.0 
32.0 
37.0 
42.0 
52.0 
62.0 
77.0 
101.0 
148.0 
195.0 
240.0 
285.0 

5 
10 

20 
30 
40 
60 
70 
85 
110 
140 
160 
190 
210 
260 
310 
390 
500 
730 
920 
1200 
1420 

5 

5 
10 
10 
15 
25 
30 
40 
55 
65 
70 
75 
85 
110 
125 
160 
200 
300 
390 
480 
570 

5 

5 
10 
15 
20 
25 
35 
45 
60 
75 
90 
110 
115 
145 
170 
210 
280 
410 
530 
650 
780 

5 
5 

5 
5 
10 
15 
15 
20 
25 
30 
35 
40 
45 
60 
70 
85 
110 
160 
215 
265 
315 

2.0  
3  0 

5.0  
7.5  
10.0  
15.0  
20.0  
25  0 

30.0  
35  0  

40.0  
50.0  

60  0...     . 

75.0  

100.0  
150.0  
200  0 

250.0  
300  0 

Western  Association  of  Electrical  Inspectors  has  compiled  the 
accompanying  table  based  on  the  code  rules  which  give  this  in- 
formation for  single-phase,  two-phase  and  three-phase  motors. 
Excerpts  from  the  report  and  the  recommendations  for  three- 
phase  motors  follow: 

No  consideration  was  given  to  limiting  the  voltage  drop  at 
the  motor.  Motors  without  a  starting  device  and  those 
operating  at  less  than  600  r.p.m.  require,  in  the  majority  of 
cases,  one  size  larger  wire  or  cable  than  the  table  calls  for. 


METHODS  USED  TO  SOLVE  SPECIAL  TROUBLES     443 

The  wires  in  the  table  are  calculated  for  two  and  one-half 
times  the  full  load  current  of  motors  up  to  30-amp.  rating 
and  for  twice  the  full-load  current  of  larger  motors. 

Wire  or  cable  sizes  for  other  types  of  continuous-duty 
induction  motors  should  be  based  on  the  following  multiples 
of  the  full-load  current:  Squirrel-cage  motors  up  to  7.5  hp. 
without  starters,  three  times  full-load  current;  squirrel-cage 
motors  with  star-delta  starting  switch,  one  and  a  half  times 
full-load  current;  wound-rotor  motors  with  resistance  in  rotor, 
one  and  a  tenth  times  full-load  current,  and  single-phase 
repulsion  motors  up  to  15  hp.,  twice,  and  single-phase  motors 
with  split-phase  starting,  three  times  full-load  current. 


CHAPTER  XVII 

MACHINE  EQUIPMENT  AND   TOOLS  NEEDED   IN  A 
REPAIR  SHOP 

The  work  that  comes  to  the  average  electrical  repair  shop 
varies  from  the  re-winding  a  fan  motor  to  the  overhauling  and 
re- winding  of  large  station-  generators.  In  the  latter  case  the 
work  must,  in  the  majority  of  cases,  be  done  at  the  location  of 
the  machine  and  calls  for  more  portable  tools  than  machine 
equipment.  To  meet  the  requirements  of  work  between  these 
limits  in  size  of  machine  there  is  needed  a  fairly  well  equipped 
machine  shop.  Most  motor  repair  and  armature  winding 
work  can  be  properly  handled  if  the  following  equipment  is 
available : 

Lathe. 

Coil- winding  lathe  heads. 

Shaper,  or  undercutting  attachment  for  lathe. 

Drill  press. 

Band  saw. 

Emery  wheel. 

Portable  hand  crane. 

Chain  block. 

Welding  outfit. 

Vices. 

Coil-pulling  machine. 

Coil-taping  machine. 

Banding  machine  or  tension  device  for  lathe. 

In  case  only  one  lathe  is  available  this  should  be  large  enough 
to  accommodate  a  good  sized  armature  and  have  a  spread 
between  head  and  tail  post  of  about  60  inches.  The  shaper 
and  band  saw  are  rather  special  machines  for  a  repair  shop 
but  very  useful.  The  former  serves  as  a  rapid  and  accurate 
method  for  undercutting  the  mica  of  a  commutator.  The 

444 


REPAIR  SHOP  EQUIPMENT  AND  TOOLS 


445 


latter  saves  a  great  deal  of  time  when  many  coil  forms  are  to 
be  cut  for  several  coil  winders. 

In  the  electrical  repair  shop  of  a  large  industrial  plant  where 
a  number  of  armatures  are  re-wound  and  repaired  the  following 
equipment  is  provided: 

1  12-inch  speed  lathe. 

1  24-inch  armature-banding  lathe. 

1  24-inch  commutator  grooving  lathe. 

1  Commutator  turning  device. 

1  Double  head  emery  wheel  grinder. 

1  3-inch  bench  vice. 

1  5-inch  bench  vice. 

2  6-inch  bench  vices. 

2     Field-winding  machines. 

2     Armature  coil-taping  machines. 

1     One-ton  electric  hoist  and  carriage. 


FIG.  275. — Armature  winders'  hand  tools.    (Numbers  refer  to  their  names  and 
uses,  page  446.) 

Armature  Winder's  Tools. — The  experienced  armature 
winder  usually  has  a  variety  of  tools  either  designed  according 
to  his  own  ideas  or  convenience  for  a  particular  job  or  for  use 
in  connection  with  armatures  of  different  sizes.  In  Figs.  275 
and  276  a  varied  assortment  of  hand  tools  and  drifts  are  shown. 
The  names  of  these  tools  and  their  uses  are  given  in  the 
following  tabulations. 


446         ARMATURE  WINDING  AND  MOTOR  REPAIR 


FIG.  276. — Drifts  used  by  armature  winders.     (Numbers  refer  to  types  and 

uses,  page  447.) 

ARMATURE  WINDER'S  HAND  TOOLS  (Fio.  275) 


Number 

in 
illustration 


Name  of  tool 


Uses  in  winding  armatures 


1 
2 

3 
4 

5  and  6 

7 

8 

9 

10 

11  and  12 


Coil  hook.  . . 
Medium  file. 


Knife  edge  file 

16-oz.  machinist's  hammer 

No.  1  and  No.  2  rawhide  mallets , 

12-oz.  machinist  hammer 

Tinsmith's  hammer 

Cold  chisel 

Coil  lifter  and  shaper'. 


Metal  drifts. 


13  and 
14 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 


15 


Two  sizes  of  wedge  drivers . 

Undercutting  tool 

Coil  scraper 

Diagonal  cutters 

Half-round  duck  bill  pliers. 

Round-nose  pliers 

Small  screw  driver 

Shoemaker's  knife 

Scissors 

Tinsmith's  shears 

6-in.  parallel  pliers 

8-in.  side  cutting  pliers 

Flat  duck  bill  pliers 

Long  metal  coil  drift 

Monkey  wrench 

Large  screw  driver 


To  lift  coils  out  of  slots. 

For  cleaning  commutator  necks  before 
soldering. 

For  trimming  edges  of  mica  segments. 

For  use  on  metal  surfaces. 

For  drifting  coils  into  slots. 

For  use  on  metal  surfaces. 

For  peening  slots  of  commutator  necks. 

For  cutting  coil  leads  to  commutator. 

For  lifting  coils  out  of  slots  and  shaping 
when  rewinding.  !*• 

For  driving  down  coil  terminals  in  com- 
mutator necks. 

For  driving  wedges  into  slots. 

For  undercutting  mica  on  commutators. 

For  scraping  cotton  insulation  off  wires. 

Special  pliers  for  rewinding  work. 

Special  pliers  for  rewinding  work. 

Special  pliers  for  rewinding  work. 

For  general  use. 

For  triming  and  cutting  slot  insulation. 

For  cutting  tape,  and  cotton  duck. 

For  cutting  tin  clips  for  banding  wires. 

For  gripping  wires  in  winding  coils,  etc. 

For  cutting  heavy  wire  coil  terminals. 

For  shaping  coils. 

For  lifting  tight  coils  out  of  slots. 

For  tightening  commutator,  etc. 

For  general  use. 


REPAIR  SHOP  EQUIPMENT  AND  TOOLS 
DRIFTS  USED  BY  ARMATURE  WINDERS  (Fie.  276) 


447 


Number 

in 
illustration 


Name  of  drift 


Particular  uses 


3,  4  and  5 


7 
8 

9,  10,  11 
and  12 


13,  14,  15, 
18,  19,  20, 
21  and  22 

16 
17  and  23 


Coil  sliders  or  guides 

Different  sizes  of  fiber  slot  drifts. 


Fiber  coil  shapers . 


Fiber  coil  drift  tapered. 
Small  metal  coil  lifter. . 
Center  punch 


Lead  lifters. 


Metal  drifts. 


Ordinary  dividers. 
Tee  slot  drifts... 


For  sliding  top  sides  of  coils  into  slots. 
For  driving  coils  into  slots.     They  vary 

in  width  and  length  according  to  size 

of  slot. 
The  curved  surfaces  are  used  to  shape 

ends  of  coils. 

For  lifting  tight  coils  in  small  machines. 
For  lifting  tight  coils  out  of  slots. 
For  marking  location  and  pitch  of  coil 

when  stripping  armature. 

For  driving  soldered  ends  out  of  com- 
mutator necks  when  stripping  arma- 
ture. 

For  seating  coil  terminals  in  commutator 
necks. 


For  laying  out  fiber  collars. 
For   partly    closed    slots    to    drive 
down  in  slots. 


oils 


Device  for  Shaping  Insulating  Cells  of  Armature  Slots. — 

For  shaping  the  fish  paper  in  making  cells  for  armature  and 
stator  slots  the  cell  shaper  shown  in  Fig.  277  is  very  useful. 
It  consists  of  two  pieces  of  wood  hinged  together  so  that  they 
will  make  a  neat  90-deg.  fold.  The  permanency  of  the  correct- 
fold  maker  is  insured  by  means  of  a 
metal  strip  attached  to  the  wood  slot. 
The  cell  shaper  is  used  by  inserting 
a  piece  of  fish  paper  in  the  opening 
between  the  two  blocks  of  wood,  which 
is  the  length  of  the  slot  plus  twice  the 
height  and  whose  width  is  the  width 
of  the  slots.  The  metal  straight-edge, 
which  is  adjustable  by  means  of  wing  nuts,  allows  the  paper 
to  be  folded  so  as  to  be  made  the  height  of  the  slot. 

Tool  for  Cutting  Cell  Lining  at  Top  of  Slot. — For  cutting  the 
projecting  insulation  of  an  armature  slot  after  the  coils  have 
been  assembled,  the  tool  shown  in  Fig.  279  is  recommended  by 
Maurice  S.  Clement  (Electrical  World,  Oct.  12,  1918).  This 


CELL  SHAP£R 

FIG.  277. — Device  for 
shaping  insulating  cells  of 
armature  slots. 


448          ARMATURE  WINDING  AND  MOTOR  REPAIR 


•S  j*» 
fl  "fl 

,,-.    <D 
P    3 


3 


REPAIR  SHOP  EQUIPMENT  AND  TOOLS 


449 


riLEHANDLE 


tool  as  well  as  those  of  Figs.  280  and  282  and  the  insulating  cell 
shaping  device  shown  in  Fig.  278  are  home-made  designs  he  has 
devised.  It  consists  of  a  piece  of  forged  steel  14  inches  long 
by  %  inch  wide  by  %Q  inch  thick  with  a  set  of  beveled  knife- 
edges  at  one  end  and  a  file  handle  at  the 
other. 

Special  Winding  Tools. — In  Fig.  280 
three  convenient  tools  are  shown  that  can 
bs  easily  made  by  any  armature  winder. 
From  left  to  right  in  the  illustration  they  FlG  279— Tool  for 
are  a  coil-taping  needle,  coil  raiser,  and  cutting  insulation  that 
wire  scraper.  The  coil-taping  needle  can  projec 
be  made  from  one  foot  of  No.  14  banding  wire  shaped  so  that 
it  can  be  used  for  taping  coils  in  closed-slot  stators.  After 
the  user  is  accustomed  to  this  device  high  speed  may  be 
attained. 

The  coil  raiser  consists  of  a  piece  of  steel,  16  inches  by  1  by 
mch  w^h  a  four-inch  one-sided  taper  on  one  end  for  strip- 


CELL  CUTTER 


*tt 

v'/SEnd 

—  r 

~jj 

•• 

) 

* 

Plan 

•£ 

A 

CO/ 

i 
rV 

LRAISEK 

L     \ 

Section 


MEDLE"  ^MRESCRAPE* 

FIG.  280. — Convenient  designs  of  coil  taping  needle,  coil  raiser  and  wire 

scraper. 

ping  open-slot  armatures  and  stators.  This  also  can  be  used 
to  good  advantage  in  removing  grounded  coils  from  a  newly 
wound  armature  or  in  raising  coils  sufficiently  to  allow  for 
insulating  weak  spots  in  the  coils,  the  main  object  in  this  case 

33 


450         ARMATURE  WINDING  AND  MOTOR  REPAIR 


REPAIR  SHOP  EQUIPMENT  AND  TOOLS 


451 


being  to  lift  out  a  tight-fitting  coil  without  damaging  the 
insulation. 

The  wire  scraper  is  very  simply  made  and  very  economical, 
because  it  eliminates  the  use  of  a  knife,  whose  life  is  short  on 
account  of  the  rough  treatment  accorded  it.  This  device  is 
made  of  spring  metal,  12  inches  by  %  inch  by  Jfe  mcn-  The 
knife-edges  can  be  sharpened  by  means  of  a  file  and  the  tool 
used  indefinitely.  A  section  the  shape  of  a  rectangle  is  cut 
from  the  metal  at  the  handle  end,  greatly  increasing  the  spring 
effect  of  the  device. 

For  driving  fiber  wedges  between  the  top  of  a  coil  and  the 
lamination  overhanging  closed-slots,  a  wedge  drift,  made  of 
a  piece  of  tool  steel,  eight  inches  by  five  inches  by  ^2  inch, 
over  which  is  fitted  a  loose-fitting  steel  sleeve,  J^g  inch  thick, 
is  very  convenient.  This  is  used  by  inserting  the  fiber  wedge 
about  y±  inch  into  the  slot;  then,  with  the  drift  pulled  back  into 
the  sleeve,  the  sleeve  is  fitted  over  the  wedge,  which  is  driven 
into  the  proper  place,  the  sleeve  holding  the  wedge  in  position. 

Repair  Tools  that  Can  be  Made  from  Old  Hack-saw  Blades. 
Seven  tools  that  have  been  made  from  old  hack-saw  blades 


FIG.  282. — Seven  armature  repair  tools  made  from  old  hack-saw  blades  that 
are  useful  around  a  motor  repair  bench. 

and  found  useful  in  the  re-winding  of  motors  are  shown  in 
Fig.  282.  A  small  wooden  case  to  stow  away  these  tools  can 
be  made  as  follows:  Cut  out  two  pieces  of  any  suitable  hard 
wood  eight  inches  long,  one  and  one-half  inches  wide  and 
]4,  inch  thick.  Then  on  the  flat  side  of  one  of  these  pieces 
cut  several  grooves,  large  enough  to  allow  a  hack-saw  blade 
to  fit  in  it  snugly.  The  two  pieces  should  then  be  nailed 


452 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


together  so  that  the  grooves  will  be  between  both  pieces. 
When  a  third  piece  of  wood,  eight  inches  long,  one  inch  wide 
and  y±  inch  thick  is  nailed  on  as  a  bottom  piece,  the 
case  is  ready  for  the  tools. 

While  each  of  the  tools  can  be  used  for  several  different 
operations,  a  few  of  the  most  important  uses  are  as  follows: 
Tools  one,  two  and  five  (from  the  right)  are  used  mostly  in 
taping  coils.  Tools  one  and  two  are  used  to  best  advantage  in 
digging  stray  drops  of  solder  out  of  the  winding  of  a  stator 
after  the  connections  have  been  soldered.  Tools  four  and  six, 
which  have  both  edges  of  the  V  sharpened  to  a  knife-edge, 
are  very  well  adapted  to  cutting  insulation  and  scraping  wires 
or  leads.  Number  seven  is  used  to  drive  wedges  into  tight 
slots.  By  setting  the  saw-teeth  on  the  wedge  and  pounding 
on  the  beveled  end  with  a  hammer,  the  teeth  are  made  to 
grip  the  wedge  and  drive  it  to  its  proper  place. 

Special  Coil-winding  Device. — For  winding  coils  of  practi- 
cally any  shape  the  special  device  shown  in  Fig.  283  has  been 


Ji  Hole  off  Center 


J^St'd.  Thread 
and  Washer 

FIG.  283. — Framework  for  use  in  winding  different  shapes  of  armature  coils. 

devised  by  Frank  Huskinson  (Electrical  World,  July  27,  1918). 
It  consists  of  an  iron  framework  held  together  with  four  bolts 
shown  at  B.  By  loosening  the  nuts  of  these  bolts  the  two 
outside  members  can  be  adjusted  for  any  width  of  coil  needed. 
To  give  the  coil  the  proper  shape,  disks  such  as  marked  C  are 
clamped  on  the  piece  D  and  the  latter  inserted  in  the  slots 
marked  A  in  the  frame.  The  view  at  E  shows  how  these 
disks  appear  when  the  bolt  D  is  in  its  proper  place  in  the  frame. 
When  forming  the  coils  the  wire  is  wound  around  six  of  the 
bolts  or  pegs  and  between  the  disks  mounted  on  them. 


REPAIR  SHOP  EQUIPMENT  AND  TOOLS 


453 


FIG.  284. — Convenient  bench  device  for  holding  the  stator  of  a  small  motor 
when  repairing  windings. 


FIG.  285. — At  the  left  a  vertical  stand  is  shown  for  use  in  mounting  the 
commutator  on  small  armatures.  At  the  right,  a  large  armature  being  wound 
with  pushed-through  coils. 


464         ARMATURE  WINDING  AND  MOTOR  REPAIR 

Steadying  Brace  for  Repairing  Small  Motors. — The  device 
shown  in  Fig.  284  has  been  devised  by  Maurice  S.  Clement 
(Electrical  Record,  December,  1918)  for  use  in  repairing  fan 
motors  or  other  small  motors  when  it  is  necessary  to  remove 
the  winding  of  the  stator  from  the  frame.  To  remove  these 
windings  from  the  frame  the  laminations  must  be  removed 
with  the  winding.  When  once  removed  it  is  very  difficult 
to  work  on  it  as  it  has  a  tendency  to  roll  away  all  the  time. 

This  brace  is  constructed  as  follows:  A  block  is  cut  as 
shown  and  bolted  to  the  bench.  One  end  of  a  length  of 
ribbon  copper  wire  about  }{$  inch  thick  and  ^  inch  wide  is 
then  screwed  to  the  end  of  the  block  and  passed  over  the  wind- 
ing which  sets  in  the  cut  out  portion  of  the  block.  The  other 
end  of  the  copper  ribbon  is  held  down  by  means  of  a  wing 
nut  placed  between  the  bolts.  A  leather  strap  is  arranged 
from  one  side  of  the  block,  over  the  copper  ribbon  and  fastened 
to  a  buckle  on  the  other  side.  This  is  a  tension  strap  and  takes 
the  slack  out  of  the  copper  ribbon. 

Tension  Block  For  Use  When  Banding  Armatures. — A  tool 
is  shown  in  Fig.  286  which  has  been  devised  by  Maurice  S. 

W*  Ifl* 

maatm     |  "' *W*-«W*|£    , 

-* — w^~^  ; 


lii^BRC 


IBREi^rCWWE 


FIG.  286. — Tension  block  for  use  when  winding  armatures. 

Clement  (Electrical  World,  Oct.  12,  1918)  for  banding  an  arma- 
ture in  a  small  repair  shop  where  a  banding  lathe  is  not  avail- 
able. By  use  of  the  device  the  armature  can  be  banded  on  an 
ordinary  winding  stand  which  is  securely  fastened  to  the  floor 
or  work  bench.  When  this  device  is  used  about  one  foot  of 
stout  line  with  a  hook  attached  to  one  end  is  made  fast  to  the 
ring  on  the  tension  block  and  hooked  to  an  eye-bolt  which  is 
set  in  the  floor  for  that  purpose.  The  spool  of  banding  wire 
is  placed  on  a  small  stand  beside  the  eye-bolt  and  the  wire  is 
passed  between  the  two  blocks  at  the  rear  end  through  the 
hole  in  the  first  wire  guide  over  the  tension  curve  and  through 


REPAIR  SHOP  EQUIPMENT  AND  TOOLS 


455 


the  second  wire  guide  hole  and  then  to  the  armature.  The 
tension  can  be  regulated  by  the  wing  nut  placed  at  the  forward 
upper  end  of  the  block.  By  screwing  down  the  wing  nut 
both  sides  of  the  block  are  brought  nearer  together,  thus 


. 

Double  ply  leather 

%"thick               i!o" 

V     * 

1 

\1 

M 

7  Brass  rivets                                         * 

5^4 

Iron  triangle 
FIG.  287. — Armature  sling  made  up  of  double-ply  leather  belting. 

narrowing  the  tension  curve  over  which  the  wire  must  pass. 
This  increases  the  tightness  of  the  band  when  a  pipe  wrench 
can  be  used  to  revolve  the  armature. 


FIG.  288. — Method  of  using  a  rope  sling  in  handling  armatures  with  a  crane. 

Armature  Sling. — For  handling  heavy  armatures  in  repair 
shops  a  sling  is  sometimes  used  instead  of  lifting  by  means  of  a 
rope  attached  to  the  ends  of  the  shaft  as  a  bale  handle  for  the 


456          ARMATURE  WINDING  AND  MOTOR  REPAIR 


crane  hook.  The  sling  prevents  the  possibility  of  springing 
the  shaft.  A  sling  which  can  be  used  for  handling  small 
armatures  is  shown  in  Fig.  287.  It  is  constructed  of  a  piece 
of  double  ply  belting  about  %  inch  thick,  10  inches  wide  and 

5%  feet  long,  provided  with  triangles 
of  steel  bar  for  the  hook  of  a  shop 
crane. 

Pinion  Puller. — A  device  which 
can  be  made  up  as  shown  in  Fig. 
289  is  convenient  for  removing 
pinion  gears  from  motor  shafts  with- 
out injury.  By  making  the  head 
sufficiently  long  and  providing  holes 
at  suitable  distances,  pinions  of 
different  sizes  can  be  removed  by 
adjusting  the  end  pieces. 

Coil-winding  Machines. — In  Fig. 

290  a  device  is  shown  which  does  not  require  the  use  of  forms 
when  winding  armature  coils.  It  consists  of  an  ad j  ustable  metal 
frame  with  jaws  which  may  be  clamped  in  various  positions  so 
as  to  make  a  form  over  which  the  wire  can  be  wound  into  the 


FIG.  289.— Convenient  de- 
vice for  removing  pinions 
from  a  motor  shaft. 


FIG.  290. — Segur  armature  coil  winder  for  winding  hair-pin  loop,  obtuse  loop 
and  rectangular  loop  coils.      (Electrical  Manufacturers  Equipment  Company.) 

With  this  device  loops  from.  3  inches  to  36  inches  can  be  wound.  By  means  of  the 
spreader  shown  in  Fig.  291  the  coil  shapes  shown  in  Fig.  292  can  be  formed  from  these 
loops. 

shape  of  the  coil  desired.     Jaws  are  provided  for  forming  hair- 
pin loop  coils,  that  is,  coils  with  parallel  sides  and  curved  ends. 


REPAIR  SHOP  EQUIPMENT  AND  TOOLS  457 

Another  set  of  jaws  provided  with  the  machine  has  four  points 
and  gives  four  curves  to  the  coil,  the  jaws  being  set  so  as  to 
form  a  rectangular  coil  with  two  angles  obtuse  and  two  angles 
acute.  These  same  jaws  may  be  set  in  another  way  so  as  to 
give  a  triangular  or  square  loop.  With  these  attachments, 
coils  of  various  shapes  can  be  wound,  some  of  the  shapes  being 
suitable  for  use  directly  in  various  types  of  motors  and  gen- 
erators and  other  coils  requiring  further  operations  on  a 
machine  called  a  spreader.  The  purpose  of  this  machine 
(Fig.  291)  is  to  form  the  coil  to  the  throw  required  when  it  is 


FIG.  291. — Coil  spreader  for  shaping  the  coils  shown  in  Fig  292  as  wound 
on  the  machine  shown  Fig.  290.  (Electrical  Manufacturers  Equipment  Com- 
pany.) 

inserted  in  the  armature  or  stator.  The  spreader  takes  the 
coil  as  it  is  wound  upon  the  winding  machine  and  opens  it 
and  twists  it  at  both  ends,  so  that  it  conforms  to  the  shape 
desired.  Some  of  the  forms  of  coils  as  wound  on  these  ma- 
chines and  as  finally  shaped  on  the  spreader  are  shown  in  the 
accompanying  illustrations,  Fig.  292. 

Coil-taping  Machines. — Field  and  armature  coils  for  me- 
dium and  larga-sized  motor  and  generators  are  wound  from 
bare  copper  wire  and  are  insulated  after  the  winding  operation 
has  been  completed.  The  material  usually  employed  for 
winding  coils  is  treated  or  untreated  tape  which  is  wound  on 
the  complete  coil  by  hand  using  rolls  of  treated  or  untreated 
tape  or  by  the  use  of  a  taping  machine.  The  taping  machine 
consists  essentially  of  a  rotating  circular  element  which  has  an 
opening,  by  means  of  which  the  coil  is  placed  inside  of  the  ring 


458          ARMATURE  WINDING  AND  MOTOR  REPAIR 


(e) 


FIG.  292. — Coils  that  can  be  shaped  on  the  spreader  shown  in  Fig.  291. 
(1)  Common  loop,  3  wires  wide  with  5  turns  that  can  be  spread  as  shown  in  (2)- 
(3)  ao-coilgroup  3  wires  wide,  wound  5  wires  vertical  and  spiral  shown  f  ormed  in  (4)- 
(5)  a  rectangular  loop  3  wires  wide,  2  turns  of  6  wires  that  can  be  shaped  to  make  a 
3-coil  bundle.  (7)  Obtuse  loop,  5  turns,  3  wires  wide  that  can  be  formed  as  shown  in 
(8)  and  spread  to  make  an  Eickemeyer  coil  like  (9).  (10)  Ribbon  copper  coil  formed  into 
a  hair-pin  loop  by  bar  bender  (Fig.  293)  and  shaped  in  coil  spreader. 


FIG.  293. — Top  view  of  a  Segur  bar  bender  for  shaping  armature  coils  made 

of  copper  strip  from  0.05  to  0.125  inch  thick  by  any  width  up  to  1H  inch. 

(Electrical  Manufacturers  Equipment  Company.) 


REPAIR  SHOP  EQUIPMENT  AND  TOOLS 


459 


(Fig.  294).  Tape  is  then  attached  by  hand  and  the  rotating 
element  which  carries  the  spool  of  tape  winds  it  symmetrically 
about  the  bundle  of  wires  forming  the  coil. 


FIG.  294. — Segur  armature  coil-taping  machine. 
(Electrical  Manufacturers  Equipment  Company.) 


FIG.  295. — Portable  commutator  slotting  machine  that  will  handle  arma- 
tures up  to  18  inch  in  diameter.     (Electric  Service  Supplies  Company.) 

Commutator-slotting  and  Grinding  Machines. — In  Fig.  295 
is  shown  a  commutator-slotting  outfit.  This  machine  consists 
of  a  frame  in  which  the  commutator  is  held  either  before  or 


460         ARMATURE  WINDING  AND  MOTOR  REPAIR 

after  the  commutator  has  been  placed  on  the  armature,  and 
a  rotating  disk  saw  mounted  on  slides  which  are  parallel  to  the 
direction  of  rotation  of  the  disk.  The  rotating  saw  is  driven 
by  belts  or  other  suitable  means  of  transmission  from  the 
source  of  power.  The  guide  or  frame  upon  which  the  saw  is 
mounted  is  adjustable  according  to  the  diameter  of  the  com- 


Fia.  296. — Commutator  slotting  maching  that  can  be  attached  to  a  standard 
engine  lathe.     (Electric  Service  Supplies  Company.) 

mutator  to  be  worked  on  and  after  the  correct  height  and  depth 
of  the  cut  have  been  ascertained,  the  frame  is  clamped  firmly 
in  position.  The  operator  then  starts  the  saw  and  moves  it 
forward  on  its  guide  by  means  of  a  lever,  so  that  it  comes  into 
contact  with  the  commutator,  thus  cutting  a  slot.  The  arma- 
ture is  moved  through  a  space  corresponding  to  the  width  of  a 
single  bar,  and  the  saw  is  again  brought  forward  cutting 


REPAIR  StiOP  EQUIPMENT  AND  TOOLS 


461 


another  slot,  and  so  the  operation  continues  until  the  commu- 
tator has  been  slotted  all  around. 

The  face  of  the  commutator  must  be  smooth  and  perfectly 
circular  so  as  to  secure  sparkless  operation  and  the  minimum 
wear  of  brushes.  This  result  is  obtained  by  grinding  the  com- 
mutator after  it  has  been  assembled  or  slotted.  The  com- 


X 


•pi 


FIQ.  297. — Combination  armature  banding  and  tension  machine.    (Electric 
Service  Supplies  Company.) 

mutator  grinder  operates  by  bringing  a  stationary  grinding 
tool  or  abrasive  material  into  contact  with  the  rotating  com- 
mutator which  is  mounted  in  its  final  position  on  the  armature, 
in  this  case  the  entire  armature  being  mounted  in  a  lathe 
and  rotated.  Another  method  is  to  apply  a  rotating  grinding 
wheel  to  the  armature.  The  commutator  grinding  or  truing 


462 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


machine  is  essentially  a  lathe  and  operates  in  the  same  manner 
as  any  other  machine  for  grinding  a  circular  surface. 

Armature-banding  Machine. — This  machine  usually  con- 
sists of  a  suitable  stand  with  bearings  upon  which  the  arma- 
ture shaft  rests,  and  some  means  for  rotating  the  armature  by 
means  of  a  belt,  chain  or  gear  drive  from  a  line  shaft  or  from 
an  individual  motor.  A  band  wire  tension  device  similar  to  the 
tension  device  employed  for  winding  coils  is  incorporated  in  the 
machine  or  is  built  as  a  separate  machine,  which  may  be  at- 
tached to  the  armature  stand.  With  an  armature  in  position, 
the  band  wire  is  attached  at  several  points  throughout  the  length 


FIG.  298. — A  machine  equipped  so  that  coil-winding,  banding  and  grinding 
operations  may  be  performed  on  it. 

of  the  armature,  one  at  each  end  and  one  or  more  at  equal  dis- 
tances between  the  ends,  and  the  machine  is  started.  The  ro- 
tation of  the  armature  causes  the  wire  to  be  wrapped  about  the 
armature,  the  tension  device  and  guide  laying  the  wires  evenly, 
to  a  width  which  is  determined  by  the  setting  of  the  guide. 

Combination  Machines. — A  combination  coil- winding, 
banding  and  grinding  machine  is  shown  in  Fig.  298.  This 
machine  combines  into  one,  the  following  tools:  (a)  a  band- 
ing machine  with  a  self-contained  tension  carriage  for  the 
band  wire,  designed  to  handle  large  or  small  railway,  locomo- 
tive or  stationary  armatures;  (6)  a  commutator  slotting  ma- 
chine with  independent  motor  drive;  (c)  a  commutator  grinding 
machine  with  independent  motor  drive;  (d)  a  commutator 


REPAIR  SHOP  EQUIPMENT  AND  TOOLS  463 

cutting  or  turning  attachment;  (e)  a  field  and  armature  coil 
plate  mounted  on  the  main  spindle  suitable  for  all  classes  of 
heavy  form  coil  winding.  This  machine  closely  resembles  a 
large  power  lathe  but  is  provided  with  the  special  attachments 
for  performing  the  work  above  outlined.  After  an  armature 
has  had  the  coils  placed  in  the  slot  and  the  leads  soldered  to  the 
commutator,  it  may  be  placed  in  this  machine  and  the  work 
completed  without  removing  the  armature  from  the  machine. 
The  banding  attachment  is  capable  of  heavy  duty,  the  rotation 
being  under  control  of  the  operator.  When  the  machine 
is  stopped  it  automatically  locks  and  prevents  slack  in  the 
band  wire  due  to  backing  motion  of  the  armature.  Uniform 
tension  is  secured  by  a  tension  device  mounted  on  the  feed 
carriage  and  traveling  with  it.  The  feed  carriage  is  moved 
along  by  means  of  a  rack  and  pinion  and  pilot  wheel  and  is 
adjustable  vertically  in  or  out. 

The  slotting  attachment  is  supported  by  a  bracket  clamped 
to  the  tail-stock  spindle  and  is  removed  by  loosening  two  cap 
screws.  The  commutator  truing  or  grinding  attachment 
consists  of  a  traveling  grinding  wheel  supported  from  the  tail 
stock  and  motor  driven.  The  casting  which  supports  the  two 
rods  carrying  the  grinding  wheel  and  the  independent  motor 
are  adjustable  along  two  other  steel  rods  projecting  backward 
from  the  tail  stock  and  bringing  the  grinding  wheel  parallel 
to  the  face  of  the  commutator.  These  rods  are  adjustable  in 
or  out  to  suit  the  length  or  location  of  the  commutator,  and 
the  grinding  wheel  is  moved  along  by  means  of  the  screw  and 
hand  wheel.  Cutting  is  done  in  both  directions  of  travel  of 
the  wheel.  Power  is  applied  either  through  belt  and  pulley 
from  a  countershaft  or  by  an  individual  motor  drive  which 
may  readily  be  installed.  The  control  is  by  means  of  a  clutch 
which  is  operated  by  a  treadle  running  the  full  length  of  the 
machine.  The  machine  is  provided  with  two  changes  of 
speed,  a  low  speed  for  banding  and  coil  winding  and  a  high 
speed  for  commutator  cutting,  grinding  and  truing.  The 
head  stock  is  provided  with  a  coil-winding  plate  to  which  may 
be  attached  forms  for  winding  coils  of  various  sizes  and  shapes, 
upon  the  same  principle  as  the  independent  coil-winding 
machines,  designed  to  do  this  work  only. 


APPENDIX 

DATA  AND  REFERENCE  TABLES 

How  to  Remember  the  Wire  Table. — The  wire  table  for  B.  &  S. 
gauge  copper  wire  has  simple  relations,  such  that  if  a  few  constants 
are  remembered  the  whole  table  can  be  constructed  mentally  with  approxi- 
mate accuracy. 

A  wire  which  is  three  sizes  larger  than  another  wire  has  half  the  resist- 
ance, twice  the  weight  and  twice  the  area.  A  wire  which  is  ten  sizes 
larger  than  another  wire  has  one-tenth  the  resistance,  ten  times  the 
weight  and  ten  times  the  area. 

No.  10  Wire  is  0.10  inch  in  diameter  (more  precisely  0.102);  it  has  an 
area  of  10,000  circular  mils  (more  precisely  10,380);  it  has  a  resistance  of 
1  ohm  per  thousand  feet  at  20° C.  (68°F.),  and  weighs  32  pounds  (more 
precisely  31.4  pounds)  per  thousand  feet. 

The  weight  of  one  thousand  feet  of  No.  5  wire  is  100  pounds. 

The  relative  values  of  resistance  (for  decreasing  sizes)  and  of  weight 
and  area  (for  increasing  sizes)  for  consecutive  sizes  are:  0.50,  0.60,  0.80, 
1.00,  1.25,  1.60,  2.00. 

The  relative  values  of  the  diameters  of  alternate  sizes  of  wire  are: 
0.50,  0.63,  0.80  1.00,  1.25,  1.60,  2.00. 

The  "mil,"  whose  value  is  one-thousandth  (0.001)  of  an  inch,  is  the 
practical  basis  for  determining  the  diameters  and  thereby  the  areas  of 
all  wires  used  as  electric  conductors.  The  diameter  being  given,  the  area 
is  obtained  by  the  well-known  rule,  "the  area  of  a  circle,  in  circular  units, 
is  equal  to  the  square  of  its  diameter,"  hence,  the  square  of  the  diameter 
of  a  wire  expressed  in  mils  equals  the  area  of  its  cross-section.  D2  =  A, 
which  area  is  expressed  in  circular  mils  or  CM;  hence,  D2  =  CM. 

Circular  Mils. — Conductors  of  large  size  are  usually  specified  in  cir- 
cular mils  as  500,000  circular  mils,  750,000  circular  mils. 

To  find  resistance,  drop  one  cypher  from  the  number  of  circular  mils; 
the  result  is  the  number  of  feet  per  ohm. 

To  find  weight,  drop  four  cyphers  from  the  number  of  circular  mils 
and  multiply  by  the  weight  of  No.  10  Wire. 

Copper  for  Various  Systems  of  Distribution. — When  the  power  trans- 
mitted, distance,  line  loss  and  voltage  of  lamps  is  constant  and  all  wires 
30  465 


466          ARMATURE  WINDING  AND  MOTOR  REPAIR 

of  each  system  are  of  the  same  size,  the  following  is  the  relationship  of 
the  copper  required. 

Copper 
System.  Required 

2  Wire,  single-phase  or  direct  current 1 . 000 

3  Wire,  single-phase  or  direct  current 0 . 375 

4  Wire,  single-phase  or  direct  current 0. 222 

4  Wire,  two-phase 1 . 000 

4  Wire,  three-phase  with  neutral 0 . 333 

3  Wire,  three-phase  Delta  ...' 0. 75 


Classification  of  Wire  Gauges. — Wire  gauges  are  known  under  a  variety 
of  names  so  that  it  is  important  to  know  the  difference  when  conditions 
require  that  the  values  of  one  gauge  shall  be  converted  into  those  of  an- 
other. The  following  is  a  classification  of  gauges  by  names  and  uses: 

Brown  &  Sharpe  (B.&  S.)  =  American  Wire  Gauge  (A.  W.  G.). 

New  British  Standard  (N.  B.  S.)  =  British,  Imperial,  English  Legal 
Standard  and  Standard  Wire  Gauge,  and  is  variously  abbreviated  by 
S.  W.  G.  and  I.  W.  G. 

Birmingham  Gauge  (B.  W.  G.)  =  Stubs,  Old  English  Standard  and 
Iron  Wire  Gauge. 

Roebling  =  Washburn  Moen,  American  Steel  and  Wire  Go's.  Iron 
Wire  Gauge. 

London  =  Old  English  (not  Old  English  Standard). 

As  a  further  complication: 

Birmingham  or  Stubs'  Iron  Wire  Gauge  is  not  the  same  as  Stubs' 
Steel  Wire  Gauge. 

Uses  of  Various  Gauges. — B.  &  S.  Gauge. — All  forms  of  round  wire  used 
for  electrical  conductors.  Sheet  copper,  brass  and  German  silver. 

U.  S.  S.  Gauge — Sheet  iron  and  steel.  Legalized  by  act  of  Congress, 
March  3,  1893. 

B.  W.  Gauge. — Galvanized  iron  wire.     Norway  iron  wire. 

American  Screw  Co.'s  Wire  Gauge— Numbered  sizes  of  machine  and 
wood  screws,  particularly  up  to  No.  14  (0.2421  inch). 

Stubs'  Steel  Wire  Gauge— Drill  rod. 

Roebling  &  Trenton — Iron  and  steel  wire.  Telephone  and  tele- 
graph wire. 

N.  B.  S. — Hard  drawn  copper.     Telephone  and  telegraph  wire. 
London  Gauge — Brass  wire. 


APPENDIX 
DIFFERENCE  BETWEEN  WIRE  GAUGES 


467 


Gauge 

No. 

Brown  & 
Sharpe's 

Old  English  or 
London 

Stubs'  or 
Birmingham 

New  British 
Standard 

0000 

0.460 

0.454 

0.454 

0.400 

000 

0.40964 

0.425 

0.425 

0.372 

00 

0  .  36480 

0.380 

0.380 

0.348 

0 

0.32495 

0.340 

0.340 

0.324 

1 

0.28930 

0.300 

0.300 

0.300 

2 

0.25763 

0.284 

0.284 

0.276 

3 

0.22942 

0.259 

0.259 

0.252 

4 

0.20431 

0.238 

0.238 

0.232 

5 

0.18194 

0.220 

0.220 

0.212 

6 

0.16202 

0.203 

0.203 

0.192 

7 

0.14428 

0.180 

0.180 

0.176 

8 

0.12849 

0.165 

0.165 

0.160 

9 

0.11443 

0.148 

0.148 

0.144 

10 

0.10189 

0.134 

0.134 

0.128 

11 

0.09074 

0.120 

0.120 

0.116 

12 

0.08081 

0.109 

0.109 

0.104 

13 

0.07196 

0.095 

0.095 

0.092 

14 

0.06408 

0.083 

0.083 

0.080 

15 

0.05706 

0.072 

0.072 

0.072 

16 

0.05082 

0.065 

0.065 

0.064 

17 

0.04525 

0.058 

0.058 

0.056 

18 

0.04030 

0.049 

0.049 

0.048 

19 

0.03589 

0.040 

0.042 

0.040 

20 

0.03196 

0.035 

0.035 

0.036 

21 

0.02846 

0.0315 

0.032 

0.032 

22 

0.025347 

0.0295 

0.028 

0.028 

23 

0.022571 

0.027 

0.025 

0.024 

24 

0.0201 

0.025 

0.022 

0.022 

25 

0.0179 

0.023 

0.020 

0.020 

26 

0.01594 

0.0205 

0.018 

0.018 

27 

0.014195 

0.01875 

0.016 

0.0164 

28 

0.012641 

0.0165 

0.014 

0.0148 

29 

0.011257 

0.0155 

0.013 

0.0136 

30 

0.010025 

0.01375 

0.012 

0.0124 

31 

0.008928 

0.01225 

0.010 

0.0116 

32 

0.00795 

0.01125 

0.009 

0.0108 

33 

0.00708 

0.01025 

0.008 

0.010 

34 

0.0063 

0.0095 

0.007 

0.0092 

35 

0.00561 

0.009 

0.005 

0.0084 

36 

0.005 

0.0075 

0.004 

0.0076 

37 

0.00445 

0.0065 

38 

0.003965 

0.00575 

39 

0.003531 

0.005 

40 

0.003144 

0.0045 

468 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


EQUIVALENTS  OF  WIRE  SIZES  (B.  &  S.  GAUGE) 


0000  =  2 

No.  0=4 

No.  3  =  8 

No.  6  =  16 

No.  9  =  32 

No.  12  =  64 

No.  15 

000  =  2 

No.  1=4 

No.  4  =  8 

No.  7  =  16 

No.  10  =  32 

No.  13  =  64 

No.  16 

00  =  2 

No.  2=4 

No.  5  =  8 

No.  8  =  16 

No.  11  =  32 

No.  14  =  64 

No.  17 

0  =  2 

No.  3  =  4 

No.  6  =  8 

No.  9  =  16 

No.  12  =  32 

No.  15  =  64 

No.  18 

1  =  2 

No.  4=4 

No.  7  =  8 

No.  10  =  16 

No.  13  =  32 

No.  16  =  64 

No.  19 

2  =  2 

No.  5  =  4 

No.  8  =  8 

No.  11  =  16 

No.  14  =  32 

No.  17  =  64 

No.  20 

3  =  2 

No.  6  =  4 

No.  9  =  8 

No.  12  =  16 

No.  15  =  32 

No.  18  =  64 

No.  21 

4=2 

No.  7=4 

No.  10  =  8 

No.  13  =  16 

No.  16  =  32 

No.  19  =  64 

No.  22 

5  =  2 

No.  8  =  4 

No.  11  =  8 

No.  14  =  16 

No.  17  =  32 

No.  20  =  64 

No.  23 

6  =  2 

No.  9=4 

No.  12  =  8 

No.  15  =  16 

No.  18  =  32 

No.  21  =  64 

No.  24 

7  =  2 

No.  10  =  4 

No.  13  =  8 

No.  16  =  13 

No.  19  =  32 

No.  22  =  64 

No.  25 

8  =  2 

No.  11  =  4 

No.  14  =  8 

No.  17  =  16 

No.  20  =  32 

No.  23  =  64 

No.  26 

9  =  2 

No.  12  =  4 

No.  15  =  8 

No.  18  =  13 

No.  21  =  32 

No.  24  =  64 

No.  27 

10  =  2 

No.  13  =  4 

No.  16  =  8 

No.  19  =  13 

No.  22  =  32 

No.  25  =  64 

No.  28 

11  =  2 

No.  14  =  4 

No.  17  =  8 

No.  20  =  16 

No.  23  =  32 

.No.  26  =  64 

No.  29 

12  =  2 

No.  15  =  4 

No.  18  =  8 

No.  21  =  16 

No.  24  =  32 

No.  27  =  64 

No.  30 

13  =  2 

No.  16  =  4 

No.  19  =  8 

No.  22  =  16 

No.  25  =  32 

No.  28 

It  =  2 

No.  17  =  4 

No.  20  =  8 

No.  23  =  16 

No.  26  =  33 

No.  29 

15  =  2 

No.  18  =  4 

No.  21  =  8 

No.  24  =  16 

No.  27  =  32 

No.  30 

16  =  2 

No.  19  =  4 

No.  22  -  8 

No.  25  =  16 

No.  28 

17  =  2 

No.  20  =  4 

No.  23  =  8 

No.  26  =  16 

No.  29 

18  =  2 

No.  21  =  4 

No.  24  =  8 

No.  27  =  16 

No.  30 

19  =  2 

No.  22  =  4 

No.  25  =  8 

No.  28 

20  =  2 

No.  23  =  4 

No.  26  =  8 

No.  29 

21  =  2 

No.  24  =  4 

No.  27  =  8 

No.  30 

General  Wiring  Formula  For  Alternating-  and  Direct-current  Circuits. 
The  following  general  formula  may  be  used  to  determine  the  size  of 
copper  conductors,  volts  loss  in  lines,  current  per  conductor,  and  of  cop- 
per per  circuit  or  any  system  of  electrical  distribution. 


Area  of  conductor,  circular  mils  = 


Volts  loss  in  lines 


Current  in  main  conductors  = 


Pounds  copper 


D  X  W  X  C 
P  X  E* 

PXEXB 
100 

W  XT 
E 

D*  X  W  X  C  X  A 
P  X  #X1,000,000 


W  =  Total  watts  delivered. 

D  =  Distance  of  transmission  (one  way)  in  feet. 

P  =  Loss  in  line  in  per  cent,  of  power  delivered,  that  is  of  W. 

E  =  Voltage  between  main  conductors  at  receiving  or  consumer's 

end  of  circuit. 
For  continuous  current.  C  =  2160,  T  =  1,  B  =  1,  and  A  =  6.04 


APPENDIX 


469 


System 

Value 
of  A 

Value  of  C 

Value  cf  T 

Per  cent.,  power  factor 

Per  cent.,  power  factor 

100 

95 

90 

85 

80 

100 

95 

90 

85 

80 

Single-phase  
Two-phase  (4  wire)  
Three  -phase  (3  wire)  .  . 

6.04 
12.08 
9.06 

2160 
1080 
1030 

2400 
1200 
1200 

2660 
1330 
1330 

3000 
1500 
1500 

3380 
1690 
1690 

1.00 
0.50 
0.58 

1.05 
0.53 
0.61 

1.11 
0.55 
0.64 

0.17 
0.59 
0.68 

1.25 
0.62 
0.72 

APPLICATION  OF  THE  FORMULA 

The  value  of  C  for  any  particular  power  factor  is  obtained  by  dividing 
2160,  the  value  for  continuous  current,  by  the  square  of  that  power  fac- 
tor for  single-phase,  and  by  twice  the  square  of  that  power  factor  for  three- 
wire,  three-phase,  or  four-wire,  two-phase. 

The  value  of  B  depends  on  the  size  of  wire,  frequency  and  power  fac- 
tor. It  is  equal  to  1  for  continuous  current,  and  for  alternating  current 
with  100  per  cent,  power  factor.  For  sizes  of  wire  given  in  the  tables  of 
wiring  constants  (pages  470  and  471),  and  other  power  factors  and  cycles 
the  values  of  B  are  given. 

The  figures  given  are  for  wires  18  inches  apart  and  are  sufficiently 
accurate  for  all  practical  purposes  provided  the  displacement  in  phase 
between  current  and  emf .  at  the  receiving  end  is  not  very  much  greater 
than  that  at  the  generator;  in  other  words,  provided  that  the  reactance 
of  the  line  is  not  excessive,  or  the  line  loss  unusually  high.  For  example, 
the  constants  should  not  be  applied  at  125  cycles  if  the  largest  conductors 
are  used  and  the  loss  20  per  eent.  or  more  of  the  power  delivered.  At 
lower  frequencies,  however,  the  constants  are  reasonably  correct  even 
under  such  extreme  conditions.  They  represent  about  the  true  values 
at  10  per  cent,  line  loss,  are  close  enough  at  all  losses  less  than  10  per 
cent.,  and  often,  at  least  for  frequencies  up  to  40  cycles,  close  enough  for 
eren  much  larger  losses.  When  the  canductors  of  a  circuit  are  nearer 
each  other  than  18  inches,  the  volts  loss  will  be  less  than  given  by  the 
formula,  and  if  close  together,  as  with  multiple  conductor  cable,  the  loss 
will  be  only  that  due  to  resistance. 

The  value  of  T  depends  on  the  system  and  power  factor.  It  is  equal 
to  1  for  continuous  current  and  for  single-phase  current  of  100  per  cent, 
power  factor. 

The  value  of  A  and  the  weights  of  the  wires  in  the  table  are  based  on 
0.00000302  pound  as  the  weight  of  a  foot  of  copper  wire  of  1  circular  mil 
area. 

In  using  the  above  formula  and  constants,  it  should  be  particularly 
observed  that  P  stands  for  the  per  cent,  loss  in  the  line  of  the  delivered 


470         ARMATURE  WINDING  AND  MOTOR  REPAIR 

power,  not  for  the  per  cent,  loss  in  the  line  of  the  power  at  the  generator; 
and  that  E  is  the  potential  at  the  end  of  the  line  and  not  at  the  generator. 

When  the  power  factor  cannot  be  more  accurately  determined,  it  may 
be  assumed  to  be  as  follows  for  any  alternating  system  operating  under 
average  conditions:  Incandescent  lighting  and  synchronous  motors, 
95  per  cent. ;  lighting  and  induction  motors  together,  85  per  cent.;  induc- 
tion motors  alone,  80  per  cent. 

In  continuous  current,  three-wire  systems,  the  neutral  wire  for  feeders 
should  be  made  of  ^  the  section  obtained  by  the  formula  for  either  of  the 
outside  wires.  In  both  continuous  and  alternating-current  systems,  the 
neutral  conductor  for  secondary  mains  and  house  wiring  should  be  taken 
as  large  as  the  other  conductors. 

The  three  wires  of  a  three-phase  circuit  and  the  four  wires  of  a  two- 
phase  circuit  should  be  made  all  the  same  size,  and  each  conductor  should 
be  of  the  cross-section  given  by  the  first  formula. 


GENERAL  WIRING  DATA  FOR  FORMULA  FOR  DIRECT-  AND  ALTERNATING- 
CURRENT  CIRCUITS — 25  AND  60  CYCLES 


Size 
of  wire 
B.   &  S. 

Area 
wire 
cir. 
mils 

Wt.  Ibs. 
bare 
wire 
per 
1000  ft. 

Resist- 
ance 
ohms 
per 
1000  ft. 

at  2o°ri 

Value  of  B  for  formula  (page  468) 

Size 
of 
wire 
B.     & 

S. 

25  cycles 

40  cycles 

Per  cent,  power  factor 

Per  cent,  power  factor 

at  4\j  iv. 

95 

90 

85 

80 

95 

90 

85 

80 

0000 

211,600 

640.73 

0.04879 

1.23 

.29 

1.33 

.34 

1.52 

1.53 

.61 

1.67 

0000 

000 

167,805 

508.12 

0.06154 

1.18 

.22 

1.24 

.24 

.40 

1.41 

.48 

1.51 

000 

00 

133,079 

402.97 

0.07758 

1.14 

.16 

1.16 

.16 

.25 

1.32 

.35 

1.37 

00 

0 

105,560 

319.00 

0.09775 

1.10 

.11 

1.10 

.09 

.19 

1.24 

.26 

1.26 

0 

1 

83,694 

253.43 

0.1234 

1.07 

.07 

1.05 

.03 

.14 

1.17 

.18 

1.17 

1 

2 

66,373 

200.98 

0.1556 

.05 

.01 

1.02 

.11 

1.12 

.12 

1.10 

2 

3 

52,633 

159.38 

0.1962 

.03 

.02 

1 

.07 

1.08 

.07 

1.05 

3 

4 

41,742 

126.40 

0.2473 

.02 

1 

.05 

1.06 

.03 

1 

4 

5 

33,102 

100.23 

0.3120 

1 

1 

.03 

1.01 

1 

5 

6 

26,250 

79.49 

0.3934 

1 

1 

.02 

1 

1 

6 

7 

20,816 

63.03 

0.4959 

1 

1 

1 

.01 

1 

1 

7 

8 

16,509 

49.99 

0.6250 

1 

1 

1 

1 

I 

8 

9 

13,090 

39.60 

0.7886 

1 

1 

1 

1 

9 

10 

10,382 

31.40 

0.9940 

* 

1 

1 

1 

1 

10 

APPENDIX 


471 


GENERAL  WIRING  DATA  FOR  FORMULA  FOR  DIRECT-  AND  ALTERNATING- 
CURRENT  CIRCUITS — 60  AND  125  CYCLES 


Size 

of  wire 
B.  &  S. 

Area 
wire 
cir. 
mils. 

Wt.  Ib. 
bare 
wire 

iooerft. 

Resist- 
ance 
ohms 

1000  ft. 
at  20°C. 

Value  of  B  for  formula  (page  468) 

Size 
of 
wire 
B.    & 

S. 

60  cycles 

125  cycles 

Per  cent.,  power  factor 

Per  cent.,  power  factor 

95 

90 

85 

80 

95 

90 

85 

80 

0000 

211,600 

640.73 

0.04879 

.62 

.84 

1.99 

2.09 

2.35 

2.86 

3.24 

3.49 

0000 

000 

167,805 

508.12 

0.06154 

.49 

.66 

1.77 

1.95 

2.08 

2.48 

2.77 

2.94 

000 

00 

133,079 

402  .  97 

0.07758 

.34 

.52 

1.60 

.66 

.86 

2.18 

2.40 

2.57 

00 

0 

105,560 

319.00 

0.09775 

.31 

.40 

1.46 

.49 

.71 

.96 

2.13 

2.25 

0 

1 

83,694 

253.43 

0.  1234 

.24 

.30 

1.34 

.36 

.56 

1.75 

1.88 

1.97 

1 

2 

66,373 

200.98 

0.  1556 

.18 

.23 

1.25 

.26 

.45 

.60 

1.70 

1.77 

2 

3 

52,633 

159.38 

0.  1962 

.14 

.17 

1.18 

.17 

.35 

.46 

1.53 

1.57 

3 

4 

41,742 

126.40 

0.2473 

.11 

.12 

1.11 

.10 

.27 

.35 

.40 

1.43 

4 

5 

33,102 

100.23 

0.3120 

.08 

.08 

1.06 

.04 

.21 

.27 

.30 

1.31 

5 

6 

26,250 

79.49 

0.3934 

.05 

.04 

1.02 

.16 

.20 

.21 

1.21 

6 

7 

20,816 

63.03 

0.4958 

.03 

.02 

1 

.12 

.14 

.14 

1.13 

7 

8 

16,509 

49.99 

0.6250 

.02 

1 

.09 

.10 

.09 

1.07 

8 

9 

13,090 

39.60 

0.7886 

1 

.06 

.06 

.04 

1.02 

9 

10 

10,382 

31.40 

0.9940 

1 

.04 

.03 

1 

10 

Amperes  in  Alternating-current  Circuits. — The  following  tables  give 
the  amperes  per  lead  wire  per  kilowatt  for  single-phase  and  three-phase 
balanced  loads.  The  single-phase  table  can  be  used  for  two-phase 
balanced  loads  by  using  a  current  value  corresponding  to  twice  the  stated 
potential  of  the  circuit  or  by  dividing  the  current  value  at  the  potential 
of  the  circuit  by  two.  That  is,  each  wire  of  a  two-phase  circuit  carries 
one-half  of  the  current  indicated  at  the  load  specified.  These  tables 
show  the  value  of  the  current  at  power  factors  varying  from  unity  to 
70  per  cent.  The  power  of  any  circuit  in  kilowatts  can,  therefore,  be 
computed  by  dividing  the  reading  of  the  ammeter  by  the  tabulated  value 
corresponding  to  the  measured  power  factor  and  voltage  of  the  circuit. 
These  values  are  correct  only  for  a  balanced  load  (and  there  is  generally 
a  slight  unbalancing  of  the  loads  on  the  phases),  but  the  table  is  useful  in 
computing  the  sizes  of  wire  required  for  transmission  purposes. 

This  table  was  derived  from  the  following  formulas : 

For  single-phase  circuits:  Amperes  per  wire  =  watts  •*•  (volts  X 
power  factor). 

For  three-phase  circuits:  Amperes  per  wire  =  total  watts  -r  (volts 
between  wires  X  power  factor  X  A/S)- 

For  two-phase  circuits:  Amperes  per  wire  =  total  watts  -i-  (volts 
between  wires  of  one  phase  X  power  factor  X  2). 

In  making  the  computations  the  number  of  watts  was  assumed  as 
1000,  and  the  amperes  were  computed  for  various  values  of  emf.  to  a 


472 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


sufficient  number  of  decimal  places  to  insure  accuracy.  The  tables  were 
then  extended  by  multiplication  and  division.  If  desired,  these  tables 
can  be  further  extended  to  cover  voltages  outside  of  their  limits  by  using 
the  tabular  values  corresponding  to  potentials  of  one-tenth  (or  10  times) 
the  desired  potential,  care  being  used  to  shift  the  decimal  point  in  the 
proper  direction. 

The  values  for  intermediate  power  factors  can  be  approximated  from 
the  tables.  For  lower  power  factors,  the  value  of  the  current  for  unity 
power  factor  can  be  divided  by  actual  power  factor  of  the  circuit  or 
multiplied  by  the  reciprocal  of  this  power  factor. 


SINGLE-PHASE  CIRCUITS — AMPERES  FOR  ONE  KILOWATT  AT  DIFFERENT 
POWER  FACTORS 

Power  factor  in  per  cent, 


Volts 

100 

95 

90 

85 

80 

75 

70 

110 

9.0909 

9  .  5693 

10.  1010 

10.6952 

11.3636 

12.1211 

12.9870 

220 

4.5455 

4.7847 

5.0505 

5.3476 

5.6819 

6.0606 

6.4936 

440 

2.2727 

2  .  3923 

2.5252 

2.6738 

2.8409 

3  .  0303 

3  .  2467 

550 

1.8182 

1.9139 

2.0202 

2.1390 

2.2728 

2.4242 

2.5974 

1100 

0.9091 

0.9569 

1.0101 

0.0695 

1  .  1364 

1.2121 

1  .  2987 

2200 

0.4545 

0.4785 

0.5050 

0.5348 

0.5682 

0.6061 

0.6494 

3300 

0.3030 

0.3190 

0.3367 

0.3565 

0.3788 

0.4040 

0.4329 

6600 

0.1515 

0.1595 

0.1684 

0.1783 

0.1894 

0.2020 

0.2165 

11000 

0.0909 

0.0957 

0.1010 

0.1070 

0.1136 

0.1212 

0.1299 

THREE-PHASE  CIRCUITS — AMPERES  PER  WIRE  FOR  ONE  KILOWATT  AT 
DIFFERENT  POWER  FACTORS 

Power  factor  per  cent. 


Volts 

100 

95 

90 

85 

80 

75 

70 

110 

5.2486 

5.5249 

5.8319 

6.1749 

6.5608 

6.9982 

7.4980 

220 

2.6243 

2  .  7624 

2.9159 

3  .  0874 

3.2804 

3.4992 

3  .  7490 

225 

2.5660 

2.7010 

2.8511 

3.0188 

3.2075 

3.4213 

3.6657 

440 

1.'3122 

1.3812 

1.4579 

1  .  5437 

1  .  6402 

1  .  7495 

1  .  8745 

550 

1.0497 

1  .  1050 

1  .  1664 

1  .  2350 

1.3121 

1.3996 

1.4996 

1100 

0.5249 

0.5525 

0.5832 

0.6175 

0.6561 

0.6998 

0.7498 

2200 

0.2624 

0.2762 

0.2916 

0.3087 

0.3280 

0.3499 

0.3749 

3300 

0.1749 

0.1842 

0.1944 

0.2058 

0.2187 

0.2333 

0.2499 

6600 

0.0875 

0.0921 

0.0972 

0.1029 

0.1093 

0.1167 

0.1249 

11000 

0.0525 

0.0552 

0.0583 

C.0617 

0.0656 

0.0700 

0.0750 

APPENDIX 


473 


MINIMUM  SIZE   WIRE   FOR   MOTOR  SERVICES — WHEN   CONCEALED   OR 
PARTLY  CONCEALED  WIRES  ARE  USED 


HP. 

Size  wire  B.  &  S.  gauge 

110  volts 

220  volts 

550  volts 

X 

14 

14 

14 

1 

14 

14 

14 

2 

12 

14 

14 

3 

10 

14 

14 

4 

8 

12 

14 

5 

6 

10 

14 

7K 

4 

8 

14 

10 

3 

6 

12 

15 

0 

5 

10 

20 

00 

3 

8 

25 

000 

1 

6 

30 

0000 

0 

5 

40 

00 

3 

50 

.... 

000 

2 

60 

.... 

0000 

1 

70 

.... 

.... 

0 

80 

.... 

.... 

00 

90 

000 

100 



...-v  ;  — 

0000 

VALUES  OF  FIELD  CURRENT  IN  DIRECT-CURRENT  GENERATORS 

It  has  been  found  that  a  fair  average  for  the  field  amperes  of  different 
sizes  of  generators  is  as  follows : 


Kw.... 

1 

5 

10 

20 

30 

50 

75 

100 

Per  cent  

8 

6 

5 

4 

3  5 

3 

3 

2  75 

The  field  current  (expressed  as  a  percentage  of  full-load  current  on 
lines)  is  determined  with  all  of  the  resistance  cut  out,  that  is,  with  the 
rheostat  on  the  first  notch. 


474         ARMATURE  WINDING  AND  MOTOR  REPAIR 


•ui  % 


•ON  8JTA1 


-ON 


.2  a 


JO  '0N 


jo 


•edray 


V*  V*  V*  V*  V*  V*  \*  \*  \*  V*  \*  V*  V*  V*  V*  V*  V*  v* 
W\  »\  WN  WN  «N  «^  "X  «*X  WX  «X  "X  «*X  WN  «\  «\  rt\  »\  cK 


jo 


Bl 


jo 


<*   <N   O 


.2=8 


•sdray 


c.t 


\^|i  \*  v*  \«c 

«\  M\  w\  «\ 


jo 


•sdray 


II 


•ui  ' 


jo -ON 


§8 


APPENDIX 


475 


hes 


-HOOOOC5 


OOMOOM          OOOOOfN          O?OIN(N 

•*t>O-*O5  (Nt^<NOO»O  -*«)IOOJ 

—  II-IT-I  ININCOPO-^  "501^00 


OOOOOOCO-*  00«ilNOO 

<N   00    CD   »O    CO          00   CO    <-*    CN 

coec^iocD        t^Oii-ieo 


00    <N    0    Tjn    Tjl  O    00    00    O5    CO  00    t>^    O5    CO 

-Ci-HlNCC  ^M<»CiCOOO  O5-tCOCO 


a3nT?3  -g  y  -g 


uorjBjnsui 


•fOOOOCOOO 
CO    •<**    CO    OS    W 


W  CO    O5    CO    l^     CO  OS    CO    Hi 

rH  *-ii-ie^<NC«3  COrtfUS 


'M-f-^OOM 


00   O   (N    C> 

SS  §  55  S 


(MCOOOIN 


i-i  (NCO—iO(N  O5OCOO 


•-«O<Ni-ieO          <N<N 
<N    CO    •*'    CO   00  O    <N 


i- 

V. 

II 

Ja  o 

IJ 

gs 

«     8 

5? 


nd  a  number  equal  to  or  greater  than  the  given  hp. 
mber  of  the  smallest  wire  permissible.  For  other  e 
for  0.75,  and  0.78  for  0.70  per  cent,  efficiency. 


conditions  of  wiri 
,  will  be  found  th 
85;  0.89  for  0.80; 


nder 
the  he 
by  1.06 


II 


476 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


aximum  hp.  allowed  on  wires  according  to  Na 
ities.  P.  F.,  0.85;  Efficiency,  0.90. 


Table  showing 
and  carrying  ca 


O»    «-H   t-    00    •<$<    N.    OS 

£3  S  S  2  ?3i  <N  co 


SS^ggg 


a3ntj3  -g  35, 


ing 
ties 


«  2- 
og 


uoi^jnsut 

' 


^H     ^H     —  1     (N 


*H    rt    (N    (N    CO 


APPENDIX 


477 


rHOO 


,-H    rH    ^H    r-l    (N    <N    CC 


T-iOO 


'-iM>COO(N«Oi-c 
_    _   ^    ^H   IN    (N    CO 


tfl 


c   2  <B  5    • 
S3  •*<§ 

I*  "a  *3 

0>   ,0     W     Q.I3 


ft*  * 


1^1* 

2       Q  *&  i 


UI4 

2   >>5   » 


fl    a, 

^^^^^ 
*"  "^s    §   *" 

ic  o  oq  S  e 


|I|>1 

5!!i 

>  ^  *  -a  *" 

—    T3   ^  "O 

te-2. «     -  J 

o   >   ^   >.  •** 

*"  -  3  -  § 


llfll 


478 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


General  Motor  Data. — The  following  table  gives  the  horsepower,  vol- 
tage and  speed  of  standard  motors  at  various  frequencies  for  two-  and 
three-phase  operation: 

VOLTAGE,  HORSEPOWER  AND  SPEEDS  OF  STANDARD  MOTORS 


Windings 

Cycles 

Volts 

HP. 

Rpm. 

2-  and  3-phase 

60 

110,  220,  440,  550 

1,  2,3,  5,  7.5,  10,  15 

1800 

2-  and  3-phase 

60 

110,  220,  440,  550 

0.75,  1.5,  2,  3,  5,  7.5 

1200 

3-phase 

40 

220,  550 

1,1.5,  2,  3,  5, 

1200 

3-phase 

40 

220,  550 

1,2,3 

800 

3-phase 

25 

110,  220,  440 

1,  2,  3,  5 

1500 

3-phase 

25 

110,  220,  440 

1,  2,  3,  5 

750 

STANDARD   POTENTIALS   FOR   WHICH   SKELETON    FRAME    MOTORS   ARE 

WOUND 


Style  of  winding 

Cycles 

Volts 

Under  50  hp. 

50  hp. 
and  above 

3-  and  2-phase 

60 
40 
25 

220 
220 
220 

440-2200 
550 
440 

3-phase           

3-phase                

Standard  motors  will  develop  considerably  more  torque  than  that  given 
at  the  rated  speed  and  voltage,  so  that  there  is  ample  margin  to  carry  full 
load  or  temporary  overloads  under  ordinary  variations  of  voltage. 

The  maximum  output  of  an  induction  motor  varies  with  the  square 
of  the  voltage  at  the  motor's  terminal,  but  motors  will  give  their  rated 
output  even  with  a  drop  of  10  per  cent,  in  the  voltage,  as  their  maximum 
output  is  greatly  in  excess  of  the  rated  value.  At  the  lower  potential 
the  efficiency  and  power  factor  will  be  increased  at  light  loads;  the  full 
load  values,  however,  are  usually  somewhat  lower. 

Transformer  Rating  for  Alternating-current  Motors. — For  the  larger 
motors  the  capacity  of  the  transformers  in  kilowatts  should  equal  the  out- 
put of  the  motor  in  hp.  Small  motors  should  be  supplied  with  a  somewhat 
larger  transformer  capacity,  especially  if,  as  is  desirable,  they  are  expected 
to  run  most  of  the  time  near  full  load,  or  even  at  slight  overload.  Trans- 
formers of  less  capacity  than  those  given  in  the  following  table  should 
not  be  used  even  when  a  motor  is  to  be  run  at  only  partial  load. 

For  the  operation  of  industrial  motors,  from  three-phase  systems,  three 
single-phase  units  or  one  three-phase  unit  are  recommended,  although, 
if  desired,  two  single-phase  transformers  may  be  used.  The  use  of 
the  three-phase  transformer  greatly  reduces  the  space  required  and  makes 


APPENDIX 


479 


Phase  B 


Taps 


FIG.  299. — Connections    for    a   2-phase    auto-starter    for    a    4-wire  circuit. 


Generator 


Phase  A        Phase  B 


Taps 


FIG.  300. — Connections    for    a    2-phase    auto-starter    for    a    3-wire  circuit. 


Tape 


FIG.  301. — Connections  for  a  3-phase  auto-starter. 


480         ARMATURE  WINDING  AND  MOTOR  REPAIR 


the  wiring  very  simple,  while  the  only  advantage  gained  in  using  three 
single-phase  transformers  rather  than  a  three-phase  transformer  is  that 
in  the  case  of  one  transformer  burning  out,  the  other  two  may  be  used  to 
operate  the  motor  at  reduced  load. 

RATINGS  OF  TRANSFORMERS  REQUIRED  FOR  INDUCTION  MOTORS 


Size  of  motor 
hp. 

Kilowatts  per  transformer 

Two  single-phase 
transformers 

Three  single-phase 
transformers 

One  three-phase 
transformer 

1 

0.6 

0.6 

2 

1.5 

1.0 

2.0 

3 

2.0 

1.5 

3.0 

5 

3.0 

2.0 

5.0 

7^ 

4.0 

3.0 

7.5 

'10 

5.0 

4.0 

10.0 

15 

7.5 

5.0 

15.0 

20 

10.0 

7.5 

20.0 

30 

15.0 

10.0 

30.0 

50 

25.0 

15.0 

50.0 

75 

40.0 

25.0 

75.0 

100 

50.0 

30.0 

100.0 

RATING  OF  TRANSFORMERS  FOR  THREE- AND  TWO-PHASE  INDUCTION 
MOTORS  ON  VARIOUS  CIRCUITS 


Single-phase  transformer  voltages 


jjeiiverea 
voltage  of 
circuit 

110-volt  motor              % 

220-volt  motor 

Primary 

Secondary 

Primary 

Secondary 

1100 

2200 

1100 

2200 

122 

122 

1100 

2200 

244 
244 

SIZE  OF  WIRES  FOR  SINGLE-PHASE  MOTORS 


220  volts 


Jlp. 

Full-load  current-amp. 

Size  of  wire  —  B.  &  S.  gauge 

1 

6 

14 

2 

11 

12 

3 

16 

10 

4 

22 

8 

5 

26 

6 

APPENDIX 


481 


SIZE  OF  WIRES  OF  DIRECT-CURRENT  MOTORS 


Horsepower 

220  Volts 

Full  load  current, 
amp. 

Size  of  wire,  mains, 
B.  &  S.  gauge 

fize  of  wire,  branches, 
B.  &  8.  gauge 

1.0 

4 

14 

14 

2.0 

8 

14 

14 

3.0 

12 

14 

14 

4.0 

15 

14 

12 

5.0 

19 

12 

10 

7.5 

28 

8 

8 

10.0 

38 

6 

6 

12.5 

47 

6 

4 

15.0 

56 

5 

4 

17.5 

65 

4    , 

3 

20.0 

75 

3 

1 

25.0 

94 

1 

0 

30.0 

113 

0 

2/0 

35.0 

131 

2/0 

3/0 

40.0 

150 

2/0 

4/0 

45.0 

169 

3/0 

4/0 

50.0 

188 

4/0 

250,000  C.M. 

55.0 

206 

4/0 

300,000  C.M. 

60.0 

225 

4/0 

300,000  C.M. 

65.0 

244 

250,000  C.M. 

350,000  C.M. 

70.0 

263 

300,000  C.M. 

400,000  C.M. 

75.0 

281 

300,000  C.M. 

500,000  C.M. 

80.0 

300 

350,000  C.M. 

500,000  C.M. 

85.0 

319 

400,000  C.M. 

500,000  C.M. 

90.0 

338 

500,000  C.M. 

600,000  C.M. 

95.0 

356 

500,000  C.M. 

600,000  C.M. 

100.0 

375 

500,000  C.M. 

700,000  C.M. 

125.0 

463 

700,000  C.M. 

900,000  C.M. 

150.0 

563 

800,000  C.M. 

1,100,000  C.M. 

200.0 

750 

1,300,000  C.M. 

1,700,000  C.M. 

250.0 

938 

1,700,000  C.M. 

2-900,000  C.M. 

300.0 

1,125 

2-800,000  C.M. 

2-1,100,000  C.M. 

Column  headed  "  Size  of  wire,  branches  "  gives  size  of  wire  for  branches 
and  for  mains  supplying  one  motor  and  is  based  on  50  per  cent,  over- 
load. 

Column  headed  "Size  of  wire,  mains"  gives  size  of  wire  to  be  used  for 
mains,  but  in  no  case  must  the  size  of  these  mains  be  less  than  that  re- 
quired for  the  50  per  cent,  overload  on  the  largest  motor  such  mains 
supply. 

The  question  of  drop  is  not  taken  into  consideration  in  these  tables. 

31 


482         ARMATURE  WINDING  AND  MOTOR  REPAIR 


aT   -  M 

•-  8  a 

TtH^TiHTH^^Csq0o0o0cOOTt,^cv.)(N^|000 

|]« 

00 

1 

1 

o 

sf 

5 

g  "? 

TtHTjHT^T^TtHT^^fNfNOQOOOCOCOO^TH^COC^ 

i  „ 

jS 

i 

O10B8 

lit 

N«.locooc522g,og3..32oSoE:« 

s 

3  5)  * 

1 

£ 

«    Si 

as 

4 

•Sja 

O  Q 

K 

O  pg  CO 

<N   CM   CO   rt<    TfH    O    O 

§ 

ffl-°« 

S8 

(N    CO 

03 

• 

| 

1 

ire  main 
.  gauge 

0 

<N 

|2 

(N   CO   CO 

S 

r 

'"  V 

Oj"fl       • 

r2     2    a 

X)T_22^coco^io^bI§oi2c5^ioc§ 

^' 

| 

OOOOOiCOiOO^OOOOOOOOOOO 

1 

5 

APPENDIX 


483 


000   .00.  00 


d  9  O,  Q  d 


o  o 

o  o 


CO 


1-Hr-lOOOOOO 
CO    ^^ 


. 

odd 


OOOOOOOOQQOO 


d  d  d  d     d  d 


p  o  o  o  o 

CO     ^     Tt<     Tj<     Tt< 


Ifl 

^i  tO  Is* 


OOOOOOOOOOOO 

o«oo»ooidotocJooc5 

t>.t>.QOOOO5OiO<M»OO»OO 

i— I     i— I     rH     C<l    O<    CO 


484         ARMATURE  WINDING  AND  MOTOR  REPAIR 
WIRING  DATA  FOR  DIRECT-CURRENT  MOTORS,  115  VOLTS 


Horsepower 

Approx. 
full 
load 
current, 
amp. 

Size  of 
fuses,  amp. 

Size  of 
fused  switches, 
amp. 

Size  of  wire, 
B.  &  S.  gauge 

Size  of  conduit, 
inches 

H 

2.5 

3 

30 

14 

y2 

H 

4.4 

5 

30        i 

14 

x 

i 

8.4 

10 

30 

14 

y* 

2 

17.0 

20 

30 

12 

H 

3 

23.6 

30 

30 

8 

H 

5 

38.7 

50 

60 

6 

i 

7K 

57.6 

75 

100 

4 

i 

10 

75.1 

85 

100 

2 

IK 

15 

113 

150 

200 

00 

2 

20 

151 

175 

200 

000 

2 

25 

191 

225 

400 

0000 

2 

30 

226 

275 

400 

300000 

2>i 

35 

264 

325 

400 

400000 

2K 

40 

300 

375 

400 

500000 

3 

50 

375 

*450 

500 

600000 

3 

55 

405 

*500 

500 

700000 

3H 

65 

480 

*600 

600 

900000 

4 

70 

520 

*650 

700 

1000000 

4K 

75 

555 

*700 

700 

1100000 

4K 

*When  fuse  sizes  exceed  600  amperes,  circuit  breakers  of  approved 
type  should  be  substituted. 


APPENDIX  485 

WIRING  DATA  FOR  DIRECT-CURRENT  MOTORS,   230  VOLTS 


Horsepower 

Approx. 
full 
load 
current, 
amp. 

Size  of 
fuse,  amp. 

Size  of  fused 
switches,  amp. 

Size  of  wire, 
B.  &  S.  gauge 

Size  of  conduit, 
inches 

H 

1.3 

3 

30 

14 

H 

H 

2.3 

3 

30 

14 

H 

i 

4.2 

5 

30 

14 

H 

2 

8.6 

10 

30 

14 

a 

3 

11.8 

15 

30 

12 

H 

5 

19.0 

25 

30 

10 

H 

7K 

28.6 

35 

60 

8 

H 

10 

37.6 

45 

60 

6 

i 

15 

55.0 

70 

100 

4 

i 

20 

73.8 

90 

100 

2 

1M 

25 

95 

125 

200 

0 

in 

30 

113 

150 

200 

00 

2 

35 

130 

175 

200 

000 

2 

40 

150 

175 

200 

000 

2 

50 

185 

225 

400 

0000 

2 

55 

200 

250 

400 

250000 

2 

60 

219 

275 

400 

300000 

2^ 

65 

238 

300 

400 

350000 

^ 

70 

260 

325 

400 

400000 

2K 

75 

274 

350 

400 

450000 

3 

80 

288 

375 

400 

500000 

3 

85 

308 

400 

400 

500000 

3 

90 

328 

*425 

*500 

500000 

3 

100 

368 

*450 

*500 

600000 

3 

*When  fuse  sizes  exceed  600  amperes,  circuit  breakers  of  approved 
type  should  be  substituted. 


486 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


WIRING  DATA  FOR  DIRECT-CURRENT  MOTORS,  550  VOLTS 


Horse- 
power 

Approx. 
full-load 
current-amp. 

Size  of 
fuses,  amp. 

Size  of  fused 
switches,  amp. 

Size  of  wire, 
B.  &  S.  gauge 

Size  of 
conduit, 
inches 

H 

0.52 

1 

30 

14 

H 

K 

0.94 

2 

30 

14 

H 

i 

1.80 

3 

30 

14 

K 

2 

3.50 

5 

30 

14 

y2 

3 

4.93 

6 

30 

14 

x 

5 

8.00 

10 

30 

14 

K 

7H 

12.00 

15 

30 

12 

K 

10 

15.60 

20 

30 

12 

H 

15 

22.80 

35 

60 

8 

X 

20 

30.70 

40 

60 

6 

i 

25 

40.00 

50 

60 

6 

i 

30 

47.00 

60 

60 

6 

i 

35 

55.00 

70 

100 

4 

i 

40 

62.00 

75 

100 

2 

1M 

50 

78.00 

95 

100 

2 

IK 

55 

84.00 

100 

100 

1 

IK 

60 

92.00 

125 

200 

0 

IK 

65 

98.00 

125 

200 

00 

IK 

70 

108  .  00 

150 

200 

00 

2 

75 

114.00 

150 

200 

00 

2 

80 

120.00 

150 

200 

00 

2 

85 

129.00 

150 

200 

00 

2 

90 

138.00 

175 

200 

000 

2 

100 

152.00 

200 

200 

0000 

2 

WIRING  DATA  FOR   INDUCTION    MOTORS — SINGLE-PHASE — 110   VOLTS, 
ALL   FREQUENCIES,  STANDARD  SPEEDS 


Horse- 
power 

Approx. 

full  current- 
amperes 

Size  wire 
B.  &  S. 
gauge 

Size  of 
switches 
in  amperes 

Size  of 
starting 
fuses, 
amperes 

Size  of 
running 
fuses, 
amperes 

Size  of 
conduit, 
inches 

1 

16.4 

10 

30 

25 

20 

H 

2 

24.0 

8 

60 

45 

30 

i 

3 

33.6 

6 

100 

70 

45 

1M 

4 

43.6 

4 

100 

85 

60 

IK 

5 

54.0 

4 

100 

100 

70 

IK 

7K 

80.0 

1 

200 

200 

100 

IK 

10 

106.0 

0 

200 

200 

125 

2 

APPENDIX 


487 


WIRING    DATA   FOB   INDUCTION  MOTORS — SINGLE-PHASE — 220  VOLTS, 
ALL  FREQUENCIES,  STANDARD  SPEEDS 


Horse- 
power 

Approx. 

full  current- 
amperes 

Size  of 
wire, 
B.  &  S. 
gauge 

Size  of 
switches 
in  amperes 

Size  of 
starting 
fuses, 
amperes 

Size  of 
running 
fuses, 
amperes 

Size  of 
conduit, 
inches 

1 

8.2 

14 

30 

15 

10 

X 

2 

12.0 

10 

30 

25 

15 

H 

3 

16.8 

8 

60 

35 

20 

i 

4 

21.8 

6 

60 

45 

25 

iK 

5 

27.0 

6 

60 

55 

35 

IK 

7K 

40.0 

4 

100 

75 

50 

IK 

10 

53.0 

2 

100 

100 

65 

*H 

WIRING  DATA  FOR    INDUCTION    MOTORS — THREE-PHASE — 110    VOLTS, 
ALL   FREQUENCIES,  STANDARD  SPEEDS 


Horse- 
power 

Approx. 

full  current- 
amperes 

Size  of 
wire, 
B.  &  S. 
gauge 

Size  of 
switches  in 
amperes 

Size  of 
starting 
fuses, 
amperes 

Size  of 
running 
fuses, 
amperes 

Size  of 
conduit, 
inches 

1 

6 

14 

30 

15 

9 

i* 

2 

12 

12 

30 

25 

18 

x 

3 

18 

'  10 

60 

35 

27 

H 

5 

27 

6 

100 

65 

40 

i 

7K 

39 

4 

100 

80 

58 

1H 

10 

51 

2 

200 

125 

75 

iK 

15 

75 

1 

200 

150 

112 

IK 

20 

101 

00 

400 

225 

150 

2 

25 

125 

000 

400 

250 

187 

2 

30 

150 

0,000 

400 

325 

225 

IK 

35 

175 

300.000 

400 

400 

262 

2K 

40 

210 

400,000 

500 

*500 

315 

3 

50 

246 

450,000 

500 

*500 

369 

3 

*  When  fuse  sizes  exceed  600  amperes,  circuit  breakers  of  approved 
type  should  be  substituted. 


488 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


WIRING  DATA  FOR  INDUCTION    MOTORS — THREE-PHASE — 220    VOLTS, 
ALL  FREQUENCIES,  STANDARD  SPEEDS 


Horsepower 

Approx. 
full 
current- 
amperes 

Size  of 
wire, 
B.  &.  S. 
gauge 

Size  of 
switches 
in 
amperes 

Size  of 
starting 
fuses, 
amperes 

Size  of 
running 
fuses, 
amperes 

Size  of 
conduit, 
inches 

u 

1.0 

14 

30 

5 

u 

s*± 

K 

2.0 

14 

30 



5 

/  £t 

K 

i 

3.0 

14 

30 

10 

5 

H 

2 

6.0 

14 

30 

15 

10 

H 

3 

9.0 

12 

30 

25 

15 

H 

5 

13.3 

10 

60 

35 

20 

H 

7K 

19.5 

8 

60 

45 

30 

i 

10 

25.5 

6 

100 

65 

40 

i 

15 

37.5 

4 

100 

100 

60 

1H 

20 

50.5 

2 

200 

130 

75 

IK 

25 

62  '.5 

1 

200 

150 

95 

IK 

30 

75.0 

0 

200 

200 

100 

IK 

35 

87.5 

0 

200 

200 

125 

IK 

40 

105.0 

00 

400 

225 

150 

2 

50 

123.0 

000 

400 

250 

175 

2 

75 

186.0 

300,000 

400 

400 

275 

2K 

100 

243.0 

450,000 

500 

*550 

350 

3 

150 

362.0 

800,000 

800 

*850 

*550 

3K 

200 

480.0 

1,200,000 

1,000 

*1,150 

*725 

4 

*  When  fuse  sizes  exceed  600  amperes,  circuit  breakers  of  approved 
type  should  be  substituted. 


APPENDIX 


489 


WIRING  DATA  FOR  INDUCTION    MOTORS — THREE-PHASE — 440    VOLTB, 
ALL   FREQUENCIES,  STANDARD  SPEEDS 


Horse- 
power 

Appro*, 
full  current- 
amperes 

Size  of 
wire, 
B.  &  S. 
gauge 

Size  of 
switches 
in  amperes 

Size  of 
starting 
fuses, 
amperes 

Size  of 
running 
fuses, 
amperes 

Size  of 
conduit, 
inches 

1 

1.5 

14 

30 

5 

3 

K 

2 

3.0 

14 

30 

10 

5 

y2 

3 

4.5 

14 

30 

15 

6 

H 

5 

i.7 

14 

30 

20 

10 

y* 

7^ 

9.8 

12 

30 

25 

15 

H 

10 

12.8 

10 

60 

35 

20 

H 

15 

18.8 

8 

60 

50 

30 

i 

20 

25.2 

6 

100 

75 

35 

i 

25 

31.3 

6 

100 

75 

50 

i 

30 

37.5 

4 

100 

90 

55 

IX 

35 

43.8 

4 

100 

100 

65 

i'K 

40 

52.5 

2 

200 

135 

75 

tot 

50 

61.5 

0 

200 

175 

90 

w 

75 

93.0 

00 

400 

225 

140 

2 

100 

121.5 

000 

400 

275 

175 

2 

150 

181.0 

300,000 

400 

400 

275 

w 

200 

240.0 

450,000 

400 

*550 

350 

3 

*  When  fuse  sizes  exceed  600  amperes,  circuit  breakers  of  approved 
type  should  be  substituted. 

Circuit-breakers  for  Overload  Protection  of  Motors. — There  are  two 
overload  conditions  that  may  injure  an  electric  motor.  (1)  A  continu- 
ous load  beyond  the  overload  rating  of  the  motor  and  (2)  by  an  exces- 
sive momentary  overload  Overloads  of  the  second  class  can  obviously 
be  much  larger  than  those  of  the  first  class  without  exceeding  safe  limits. 
For  example,  a  continuous  overload  of  50  per  cent,  may  injure  a  motor, 
while  the  safe  momentary  overload  may  be  three  or  four  times  the  full 
load.  If  a  motor  is  subjected  to  overloads  of  both  these  classes,  ade- 
quate overload  protection  may  be  difficult  with  either  a  circuit-breaker 
or  fuses.  Both  devices  must  be  set  for  maximum  allowable  current, 
usually  considerably  in  excess  of  the  continuous  safe  carrying  capacity 
of  the  motor,  and  a  continuous  overload  current  inside  this  limit  may 
work  injury.  Such  conditions  are  unusual,  however,  and  either  fuses 
or  a  properly  designed  circuit-breaker  will  ordinarily  afford  all  necessary 
protection. 

For  Induction  Motors. — In  case  the  squirrel-cage  induction  motors  is 
protected  by  a  circuit-breaker,  the  motor  should  be  so  connected  as  to  be 
without  overload  protection  when  the  starter  handle  is  in  the  starting 
position;  in  addition,  the  circuit-breaker  should  be  provided  with  a  time 


490         ARMATURE  WINDING  AND  MOTOR  REPAIR 

element  device  to  prevent  opening  when  the  starter  handle  is  thrown  to 
the  running  position.  Without  this  time  element  the  circuit-breaker 
when  connected  as  just  described,  must  be  calibrated  for  two  or  three 
times  the  rated  full-load  current  of  the  motor.  If  the  circuit-breaker 
must  be  effective  in  the  starting  position  it  must  be  provided  with  a  time 
element  and  be  calibrated  for  from  three  to  five  times  the  rated  full-load 
current;  with  this  calibration  no  protection  will  be  afforded  from  contin- 
uous overloads  which  might  work  injury. 

For  Motors  Carrying  a  Uniform  Load. — That  is,  motors  driving  line 
shafts,  fans,  machine  tools,  or  any  machines  which  give  them  a  fairly 
uniform  load,  carrying  maximum  continuous  rated  output  at  least  part 
of  the  time  and  possibly  full-rated  overload  for  short  periods.  In  general, 
the  overload  protection  for  such  motors  should  have  a  normal  calibration 
equal  to  125  per  cent,  of  normal  full-load  motor  current.  If  no  standard 
circuit-breaker  is  listed  for  this  rating,  the  next  larger  listed  size  should 
be  used.  A  circuit-breaker  selected  in  this  manner  will  have  a  maximum 
calibration  at  least  twice  full-load  current.  Where  overloads  greater 
than  200  per  cent,  of  normal  current  are  regularly  experienced  for  periods 
not  exceeding  two  seconds,  a  time  element  device  should  be  added  to  the 
circuit-breaker.  For  heavy  overloads  frequently  recurring  and  continu- 
ing for  periods  of  five  seconds  or  longer,  the  circuit-breaker  should  have 
a  maximum  calibration  of  at  least  10  per  cent,  in  excess  of  the  maximum 
current. 

For  Motors  Started  and  Stopped  Frequently. — Motors  on  cranes,  ele- 
vators, pumps,  air  compressors,  or  other  apparatus  requiring  frequent 
starting  and  stopping,  should  have  circuit-breakers  with  a  calibration 
of  at  least  25  per  cent,  in  excess  of  the  maximum  current  actually  required 
by  the  motor  during  any  continuous  period  of  more  than  five  seconds. 
A  time  element  device  should  be  supplied  to  care  for  greater  overloads 
lasting  less  than  five  seconds. 

Belting. — Rubber  belts  should  always  be  kept  free  from  grease  or 
animal  oils.  If  they  slip,  moisten  the  inside  of  the  belt  with  boiled 
linseed  oil.  Some  fine  chalk,  sprinkled  on  over  the  oil,  will  help  the  belt. 

Length  of  Belts. — Add  the  diameter  of  the  two  pulleys  together,  multi- 
ply by  three  and  one-seventh,  divide  the  product  by  two,  add  to  the 
quotient  the  distance  between  the  center  of  the  shafts,  and  the  product 
will  be  the  required  length. 

ORDINARILY  ACCEPTED  EQUIVALENTS  OF  BELTING 

2-ply  Rubber — Light  single  leather. 
3-piy  Rubber — Medium  single  leather. 
4-ply  Rubber — Heavy  single  leather. 
5-ply  Rubber — Light  double  leather. 
6-ply  Rubber — Medium  double  leather. 
7-ply  Rubber — Heavy  double  leather. 
8-ply  Rubber — Triple  leather. 


APPENDIX 


491 


The  thickness  of  rubber  belting  is  usually  figured  at  J-f  6  inch  per  ply. 

Horsepower  Transmitted  by  Belting. — One-inch  single  belt  moving  at 
a  velocity  of  1000  ft.  per  minute  equals  one  horsepower.  One-inch 
double  belt  moving  700  ft.  per  minute  equals  one  horsepower.  The 
horsepower  of  any  belt  equals  its  velocity  in  feet  per  minute,  multiplied 
by  its  width  and  divided  by  1000  for  single,  and  by  700  for  double  belts. 

The  following  table  is  based  on  single  belts  running  on  pulleys  of  equal 
diameter,  revolving  at  100  rpm.  The  power  transmitted  at  other  speeds 
is  in  direct  proportion.  For  double  belts,  multiply  by  ten-sevenths. 


HORSEPOWER  TRANSMITTED  BY  BELTING 


Diameter  of 
pulley  in 
inches 

Width  of  belt  in  inches 

2 

3 

4 

5 

6 

8 

10 

12 

14 

16 

18 

20 

22 

6 

.44 

.65 

.87 

1.09 

1.31 

7 

.51 

.76 

1.01 

1.27 

1.53 

8 

.58 

.87 

1.16 

1.45 

1.75 

9 

.65 

.98 

1.31 

1.64 

1.97 

10 

.73 

1.09 

1.45 

1.81 

2.18 

11 

.80 

1.20 

1.60 

2.00 

2.40 

12 

.87'!.  31 

1.75 

2.18 

2.62 

13 

.95 

1.42 

1.89 

2.36 

2.83 

14 

1.02 

1.52 

2.02 

2.53 

3.05 

15 

1.09 

1.64 

2.19 

2.73 

3.29 

16 

1.16 

1.74 

2.32 

2.91 

3.48 

17 

1.24 

1.85 

2.47 

3.09 

3.70 

18 

1.31 

1.96 

2.62 

3.27 

3.92 

19 

1.39 

2.07 

2.76 

3.45 

4.14 

20 

1.45 

2.18 

2.91 

3.64 

4.36 

22 

1.60 

2.40 

3.20 

4.00 

4.80 

24 

3.50 

4.40 

5.20 

7.0 

8.7 

10.5 

12.2 

14.0 

16.0 

17.0 

19.0 

26 

3.80 

4.70 

5.70 

7.6 

9.5 

11.3 

13.2 

15.  1 

28 

4.  10 

5.  10 

6.  10 

8.1 

10.2 

12.2 

14.3 

16.3 

30 

4.40 

5.40 

6.60 

8.7 

10.9 

13.  1 

15.3 

17.4 

19.0 

22.0 

24.0 

32 

4.70 

5.80 

7.00 

9.3 

11.6 

14.0 

16.3 

18.6 

34 

4.90 

6.20 

7.40 

9.9 

12.4 

14.8 

17.3 

19.8 

36 

5.20 

6.50 

7.80 

10.5 

13.1 

15.7 

18.3 

20.9 

24.0 

26.0 

29.0 

38 

5.50 

6.90 

8.30 

11.0 

13.8 

16.6 

19.3 

22.1 

25.0 

28.0 

30.0 

40 

5.80 

7.30 

8.70 

11.6 

14.6 

17.5 

20.4 

23.3 

26.0 

29.0 

32.0 

42 

6.10 

7.60 

9.20 

12.2 

15.3 

18.2 

21.4 

24.3 

28.0 

31.0 

34.0 

44 

6.40 

8.00 

9.60 

12.8 

16.0 

19.2 

22.4 

25.6 

29.0 

32.0 

35.0 

46 

6.70 

8.40 

10.00 

13.4 

16.8 

20.1 

23.4 

26.8 

48 

7.00 

8.80 

10.40 

14.4 

17.4 

21.0 

24.4 

28.0 

31.0 

35.0 

38.0 

50 

7.20 

9.00 

10.90 

14.6 

18.2 

21.8 

25.4 

29.0 

33.0 

36.0 

40-0 

54 

7.80 

9.80 

11.80 

15.6 

19.6 

23.6 

26.4 

31.2 

35.0 

39.0 

43.0 

60 

.... 

8.80 

10.80 

13.10 

17.4 

21.8 

26.2 

30.6 

34.8 

39.0 

44.0 

48.0 

66 

9.60 

12.00 

14  40 

19.  2 

24   0 

OQ    Q 

oo    a 

OQ    A 

4°.    O 

AO  n 

rq    f) 

72 

10.40 

13.00 

15.60 

21.0 

^^  .  u 
26.2 

^O  .  o 

31.4 

Oo  .  O 

36.6 

Oo  .  4 

41.8 

"±0  .  U 

47.0 

4O  .  U 

52.0 

)O  •  U 

58.0 

78 

11.40 

14.20 

17.00 

22.6 

ofi   4 

04     A 

OQ      0 

4^   4 

e  t     f\ 

C'7     f\ 

AO    A 

84 

.... 

12.20 

15.20 

19.40 

24.4 

^O  .  rt 

30.6 

o4  •  U 

36.4 

oy  .  o 
42.8 

fcO  .  4 

48.6 

51  *  U 

55.0 

Of  .U 

61.0 

\Z  •  I/ 

67.0 

492         ARMATURE  WINDING  AND  MOTOR  REPAIR 

Belting  Rules.  —  The  power  transmitted  by  a  belt  depends  upon  its 
width  and  its  thickness  and  the  speed  at  which  it  travels.  Hence  it  is 
customary  (as  mentioned  under  "Horsepower  Transmitted  by  Belting") 
to  express  the  transmitting  capacity  as  the  speed  in  icet  per  minute 
required  by  a  belt  one  inch  wide  to  transmit  one  horsepower.  For  single 
or  light  double  belt,  this  expression  is  usually  given  a  value  of  about 
700,  and  for  heavy  double  belt  approximately  450.  Thus  a  double  belt 
four  inches  wide,  running  at  2250  ft.  per  minute  will  transmit  20  hp. 

Roughly>  belt  speeds  should  not  exceed  one  mile  per  minute  ;  this  speed  is 
given  when  the  diameter  of  either  pulley  in  inches  multiplied  by  its  rpm. 
equals  20,000  (D  X  rpm.  =  20,000). 

Minimum  diameter  of  pulleys  for  long  life  of  heavy  belts  : 

For  double  belts  ...............................    12  in. 

For  double  belts,  extra  flexible  ...................    10  in. 

For  double  3-ply  belts  ..........................    18  in. 

hp.         126,500 
Ordinarily,  wd  =      CL-  X  —  r~ 


where  w  —  width  of  belt,  d  =  diameter  of  pulley  (both  in  inches),  and 
P  =  belt  stress  per  inch  width.     Safe  values  for  P  are  as  follows  : 

Single  belts,  above  4000  ft.  per  min.,  33  lb.;  below  4000,  46  lb.;  max. 
stress  never  over  50  lb.  Double  belts,  40  to  70  lb.  ;  3-ply  belts,  70  to  95  lb. 

With  standard  construction  and  normal  belt  stress,  the  stress  on  the 
shaft  and  bearings  is  within  safe  limits  as  long  as  d  exceeds  w.     To  find 
length  L  (length)  of  belt  when  the  pulley  diameters  D,  d  and  the  distance 
A  between  centers  are  known,  use  the  formula 
L  =  1.62  (D  +  d)  +  2A 

To  determine  distance  between  pulley  centers  use  the  formula 

A  =  K  (D  -  d} 

where  the  value  of  the  constant  K  ranges  from  3  to  4,  3.5  being  a  good 
average  value  when  the  diameter  D  is  from  4  to  6  times  that  of  d. 

The  following  precautions  should  be  observed  : 

a.  The  formula  does  not  apply  when  pulleys  are  of  nearly  the  same 
diameter;  in  this  case  the  distance  should  be  great  enough  to  allow  some 
belt  sag. 

6.  As  a  general  rule  the  distance  A  should  not  exceed  25  ft.  in  any  case. 

Rules  for  Pulley  Sizes.---The  following  formula  can  be  used  to  calculate 
sizes  of  pulleys  for  motor  drives  : 

,     DXN  DXN 

d  =  —  n  =  -  j  — 

n  d 

D  =  Diameter  of  driver. 
d  =  Diameter  of  driven  pulley. 
N  =  Revolutions  per  minute  of  driver. 
n  =  Revolutions  per  minute  of  driven  pulley. 

Speed  of  Pulleys.  —  When  the  diameter  of  the  driven  pulley  is  given, 
to  find  its  number  of  revolutions  proceed  as  follows:  Multiply  the  dia- 


APPENDIX  493 

meter  of  the  driver  by  the  number  of  its  revolutions,  and  divide  the  pro- 
duct by  the  diameter  of  the  driven.  The  quotient  will  be  the  number  of 
revolutions  of  the  driven  pulley. 

When  the  diameter  and  revolutions  of  driver  pulley  are  given,  to  find 
the  diameter  of  the  driven  pulley  that  will  make  any  given  number  of 
revolutions  in  the  same  time,  proceed  as  follows :  Multiply  the  diameter 
of  the  driver  pulley  by  its  number  of  revolutions,  and  divide  the  product 
by  the  number  of  revolutions  of  the  driven  pulley.  The  quotient  will 
be  the  diameter  of  the  driven  pulley. 

To  ascertain  the  size  of  the  driver  pulley  proceed  as  follows :  Multiply 
the  diameter  of  the  driven  pulley  by  the  number  of  revolutions  you  wish 
to  make,  and  divide  the  product  by  the  revolutions  of  the  driver  pulley. 
The  quotient  will  be  the  diameter  of  the  driven  pulley. 

Chain  Drives. — Chains  are  now  made  to  transmit  power  for  any  pur- 
pose in  sizes  varying  from  ^  hp.  at  3000  rpm.  to  3000  hp.  at  50  rpm. 
By  the  use  of  a  chain  instead  of  a  belt  or  gears,  power  may  be  transmitted 
with  a  positive  speed  ratio  on  short  centers  quietly  and  with  a  very  high 
efficiency.  Chains  should  be  lubricated  with  a  heavy  paste  grease  con- 
taining no  solid  matter  such  as  automobile  transmission  grease. 

Approximate  Weight  of  Solid  Pinions  and  Armed  Sprockets. — When 
T  is  the  number  of  teeth;  F  the  face  in  inches;  C  a  constant  in  pounds  per 
inch  in  face  per  tooth.  Then  T  X  F  X  C  =  weight  of  armed  sprocket. 
Add  25  per  cent,  if  the  sprocket  is  split  and  add  50  per  cent,  if  spring 
sprocket  and  split.  For  solid  pinions,  weight  =  C  X  T2  X  (F  +  1). 

Points  to  Consider  when  Calculating  Size  of  Chain — Morse  Chain 
Co. — When  the  number  of  teeth  equals  T  and  the  exact  outside  diameter 
is  D,  then  T  should  be  less  than  20  when  D  equals  the  pitch  diameter. 
When  T  is  more  than  20  teeth,  D  is  equal  to  the  pitch  diameter  plus  twice 
the  addendum. 

The  following  points  should  be  considered  in  this  connection: 

(1)  Use  sprockets  having  an  odd  number  of  teeth  whenever  possible. 

(2)  When  specially  authorized,  a  larger  number  of  teeth  than  shown 
may  be  cut  in  large  sprocket. 

(3)  Thickness  of  sprocket  rim,  including  teeth,  should  be  at  least  1.2 
times  the  chain  pitch. 

(4)  The  number  of  grooves  in  the  sprocket,  their  width  and  distance 
apart,  varies  according  to  pitch  and  width  of  chain. 

(5)  The  width  of  the  sprocket  should  be  ^  to  K  inch  greater  on  small 
drives,  and  K  to  ^  inch  greater  on  large  drives  than  nominal  width  of 
the  chain. 

(6)  An  even  number  of  links  in  the  chain  and  an  odd  number  of  teeth 
in  the  wheels  are  desirable. 

(7)  Horizontal    drives  preferred  with  slack  on  top  strand,  but  for 
short  drives  without  center  adjustment  slack  thould  be  on  the  bottom 
strand. 

(8)  Adjustable  wheel  centers  desirable  for  horizontal  drives  and  neces- 
sary for  vertical  drives. 


494' 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


*3 

>o 

J"^ 

^-t 

*O 

o 

o 

o  o 

cc 

|^» 

»0 

CO 

i—  ( 

1Q 

o 

1—1    O 

i—  1 

CO 

(*Ti 

| 

O5 

C^l 

l^-   CO 

WJ 

tO 

CO 

^( 

10 

»o 

00 

h^ 

t—  1 

§1 

1—  1 

rH 

CO 

c^ 

2 

Si 

O 

0   o 

CO 

? 

i 

CO 

"*    CO 

(N 

(M 

0. 

2 

^ 

10 

8 

o  o 

|o 

TH 

(N 

I—  1 

,0 

^1 

3 

CM   <M 

^ 

»o 

e 

10 

05 

9 

§ 

7 

s 

CO 

2 

1 

^^    CO 

e* 

CO 

T—  t 

>o 

i-H 

»0 

CO 

IO 

0» 

o 

§ 

§ 

§^ 

O 

c-. 

CO 

S3 

o»\ 

T—  1 

(N 

J^ 

1 

IO 

GO 

'    * 

1—1 

»0 

• 

OS 

«\ 

CO 
1—  1 

W 

1—  1 

1—  1 

1 

1 

1 

i 

-N 
iH 

i—  I 

J2 

O    CO 

xao 

CO 

T—  1 

gj 

»o 

g 

8 

0 

00 

^1 

CO 

1—  1 

i-H 

^ 

§ 

'"I 

X 

^H 

•^ 

'     ' 

i-K 

CO 

g 

I 

iO 

i-H 

OS 

10 

1 

»o 

S 

§ 

s 

O    10 
tX)  CO 

? 

i  o 

t-l 

•      (H 

| 

1 

.     ^ 

+3 
3 

o 

la    03 

! 

f-i      rt          43 
0      g          - 

! 

03 
> 

>    'S 

Gi 

^H 

3 

sl  - 

. 

C 

'g         -H 

M 

'o 

O     Cd 

$ 

228 

% 

T3 

"T3   ^"^ 

£3 

,H 

03 

*—      *Z* 

Q 

CD     1^         5 

03 

1 

02 

13 

OQ 
,  —  ' 

6 

.a" 

of  teeth  j  Small  sprocket 
Desirable  number  of  teeth 

sprockets  
Maximum  number  of  teeth  i 

ets.  (See  note  3.)  
Desirable  number  of  teeth 

sprockets  
To  find  pitch  diameter  of  wh 

ply  number  of  teeth  by  ( 
Addendum  —  For  outside  dii 

y 

CO 
i-H 

-2 

8 

3  *2 

II 
1^ 

Maximum  rpm  

Tension  per  1  ^ 
-  u  -jxu  Small  sprock 
inch  width  >  0  „ 
,  .  ,,  Small  sprocke 
chain,  Ibs.  } 

Radial  clearance  beyond 

J 

CJ 

_d 

d 

i 

-5 

s- 

1 

'5 
cr 

Constants  for  solid  pinions  . 
Constants  for  armed  sprock 

APPENDIX 


495 


(9)  Avoid  vertical  drives. 

(10)  Allow  a  side  clearance  for  chain  (parallel  to  axis  of  sprockets  and 
measured  from  nominal  width  of  chain)  equal  to  the  pitch. 

(11)  Maximum  linear  velocity  for  commercial  service,  1200  to  1600 
feet  per  minute. 

HORSEPOWER  TRANSMITTED  BY  STEEL  SHAFTING 


Diam- 
eter of 
shaft  in 
inches 

Revolutions  per  minute 

100 

125 

150 

175 

200 

225 

250 

300 

350 

400 

*#• 

1.2 

1.4 

1.1 

2.1 

2.4 

2.6 

3.1 

3.6 

4.3 

5.0 

1  Me 

2.4 

3.1 

3.7 

4.3 

4.5 

5.5 

6.1 

7.3 

8.5 

9  7 

1  Me 

4.3 

5.3 

6.4 

7.4 

8.5 

9.5 

10.5 

12.7 

14.8 

16  9 

I1  He 

6.7 

8.4 

10.1 

11.7 

13.4 

15.1 

16.7 

20.1 

23.4 

26.8 

1'Mft 

10.0 

12.5 

15.0 

17.5 

20.0 

22.5 

25.0 

30.0 

35.0 

40.0 

2  Me 

14.3 

17.8 

21.4 

24.9 

28.5 

32.1 

35.6 

42.7 

49.8 

57.0 

2   ?«6 

19.5 

24.4 

29.3 

34.1 

39.0 

44.1 

48.7 

68.5 

68.2 

78.0 

21Me 

26.0 

32.5 

39.0 

43.5 

52.0 

58.5 

65.0 

78.0 

87.0 

104.0 

2'Me 

33.8 

42.2 

50.6 

59.1 

67.5 

75.9 

34.4 

101.3 

118.2 

135.0 

3  Me 

43.0 

53.6 

64.4 

75.1 

85.8 

96.6 

107.3 

128.7 

150.3 

171.6 

3  Y\o 

53.6 

67.0 

79.4 

93.8 

107.2 

120.1 

134.0 

158.8 

187.6 

214.4 

3i  He 

65.9 

82.4 

97.9 

115.4 

121.8 

148.3 

164.8 

195.7 

230.7 

243  .  t> 

3i  Me 

80.0 

100.0 

120.0 

140.0 

160.0 

180.0 

200.0 

240.0 

280.0 

320.0 

4    #6 

113.9 

142.4 

170.8 

199.3 

227.8 

256.2 

284.7 

34J.7 

398.6 

455.6 

4'Me 

153.3 

195.3 

234.4 

273.4 

312.5 

351.5 

390.6 

468.7 

646.8 

625.0 

HORSEPOWER  TRANSMITTED  BY  SINGLE  ROPES 
(Working  strain  =  200e?2  where  d  is  diameter  of  rope) 


Diameter  of  rope  in  inches 


minute 

H 

« 

1.0 

1.25 

1.50 

1.75 

2.0 

1000 

1.24 

2.25 

3.57 

5.59 

8.02 

10.85 

14.20 

2000 

2.70 

3.84 

6.84 

10.68 

15.39 

20.93 

27.36 

2500 

3.30 

4.71 

8.38 

13.10 

18.86 

25.66 

33.54 

3000 

3.83 

5.46 

9.80 

15.39 

21.87 

29.74 

38.88 

3500 

4.30 

6.23 

10.09 

17.33 

24.94 

34.03 

44.35 

4000 

4.74 

6.83 

12.15 

18.98 

27.33 

37.17 

48.59 

4500 

5.01 

7.24 

12.89 

20.15 

29.00 

39.45 

51.57 

5000 

5.20 

7.47 

13.29 

20.76 

29.89 

40.65 

53.15 

5500 

5.29 

7.60 

13.53 

21.14 

30.43 

41.39 

54.11 

6000 

5.08 

7.32 

13.10 

20.36 

29,32 

39.77 

52.12 

6500 

4.74 

6.83 

12.13 

19.00 

27.34 

37.21 

48.63 

7000 

4.12 

5.93 

10.54 

16.47 

23.72 

32.26 

42.18 

Smallest  sheave,  diameter, 

inches 

26.00 

30.00 

42.00 

54.00 

60.00 

72.00 

84.00 

Allowable  weight,  tension 

carriage,  Ib  .  . 

80.00 

110.00 

200.00 

300.00 

450.00 

600  .  00 

750.00 

NOTE. — The  horsepower  decreases  when  the  velocity  is  above  5500  feet  on  account 
of  centrifugal  force. 


496 


ARMATURE  WINDING  AND  MOTOR  REPAIR 


s  i 

I   : 


I     1 5     5     B 
S     So     o     o 


APPENDIX 


497 


& 


I       I 


.& 


r— .     ^ 

Cu   ^ 
"^  T3 


?! 

-d  -e 


.&^ 

^1 


ii  -S 

03  .S 

S.-fl'**^! 

1  £  "2  5 15  -a  £ 

|  PH  £  PH   £    g  -g 
^  -S    S  ^    S  "S    2 

2  S  S  g  S  a  -5 

fPpfl 

—        ^        fl}        W        fl}  •  •   . 

^3  Jsl  j  s 

o  ^!  »>  t3  *>•  ?  ^ 

!>•     ?^»  10     r~>  IO   V^     w 

10  A3   ^   42   ^   «  £ 

i-H    iH    i-H    (M    C<l        •   H 

""^    F^    "T^    *^    T3    PC    '^ 
.^   .^   .21   .zi   .zi   ,s   ,ii 

>  > 


^^3333333 


^  .-§  J 


ill^i 

Slols 


^3  43  ^3  ^5 
5   -2   £   5 

ffi  S  ffi  ffi 
Ill's 

—     ^     ^-     — 

o    o    o    c^ 

a  s  s  a 

.2  .2  .25  .S 

p  p  p  p 


ii 

f-3 
If 

Q.3 


•S      "3 


1 


11 


It 

|«5  1 1 

3^21 

&  ^  6  o 


498         ARMATURE  WINDING  AND  MOTOR  REPAIR 


Some  Handy  Rules 

Diameter  of  a  circle  X  3.1416  =  Circumference. 

Radius  of  a  circle  X  6.283185  =  Circumference. 

Square  of  the  radius  of  a  circle  X  3.1416  =  Area. 

Square  of  the  diameter  of  a  circle  X  0.7854  =  Area. 

Square  of  the  circumference  of  a  circle  X  0.07958  =  Area. 

Half  the  circumference  of  a  circle  X  by  half  its  diameter  =  Area. 

Circumference  of  a  circle  X  0.159155  =  Radius. 

Sqare  root  of  the  area  of  a  circle  X  0.56419  =  Radius. 

Circumference  of  a  circle  X  0.31831  =  Diameter. 

Square  root  a  the  area  of  a  circle  X  1.12838  =  Diameter. 

Diameter  of  a  circle  X  0.86  =  Side  of  inscribed  equilateral  triangle 

Diameter  of  a  circle  X  0.7071  =  Side  of  an  inscribed  square. 

Circumference  of    a  circle  X  0.225  =  Side  of  an  inscribed  square. 

Circumference  of  a  circle  X  0.282  =  Side  of  an  equal  square. 

Diameter  of  a  circle  X  0.8862  =  Side  of  an  equal  square. 

Base  of  a  triangle  X  by  ^  the  altitude  =  Area. 

Multiplying  both  diameters  and  .7854  together  =  Area  of  an  ellipse 

Surface  of  a  sphere  X  by  %  of  its  diameter  =  Solidity. 

Circumference  of  a  sphere  X  by  its  diameter  =  Surface, 

Square  of  the  diameter  of  a  sphere  X  3.1416  =  Surface. 

Square  of  the  circumference  of  a  sphere  X  0.3183  =  Surface. 

Cube  of  the  diameter  of  a  sphere  X  0.5236  =  Solidity. 

Cube  of  the  radius  of  a  sphere  X  4.1888  =  Solidity. 

Cube  of  the  circumference  of  a  sphere  X  0.016887  =  Solidity. 

Square  root  of  the  surface  of  a  sphere  X  0.56419  =  Diameter. 

Square  root  of  the  surface  of  a  sphere  X  1.772454  =  Circumference. 

Cube  root  of  the  solidity  of  a  sphere  X  1.2407  =  Diameter. 

Cube  root  of  the  solidity  of  a  sphere  X  3.8978  =  Circumference. 

Radius  of  a  sphere  X  1.1547  =  Side  of  inscribed  cube. 

Square  root  of  (K  of  the  square  of)  the  diameter  of  a  sphere  =  Side 
of  inscribed  cube. 

Area  of  its  base  X  by  ^  of  its  altitude  =  Solidity  of  a  cone  or  pyramid, 
whether  round,  square  or  triangular. 

Area  of  one  of  its  sides  X  6  =  the  surface  of  a  cube. 

Altitude  of  trapezoid  X  %  the  sum  of  it  parallel  sides  =  Area. 


APPENDIX  499 

EQUIVALENT  VALUES  OF  CENTIGRADE  AND  FAHRENHEIT  SCALES 


Temperature 

Temperature 

Centigrade 

Fahrenheit 

Centigrade 

Fahrenheit 

0 

32 

80 

176 

5 

41 

85 

185 

10 

50 

90 

194 

15 

59 

95 

203 

20 

68 

100 

212 

25 

77 

105 

221 

30 

86 

110 

230 

35 

95 

115 

239 

38 

100.4 

120 

248 

40 

104 

125 

257 

42 

107.6 

130 

266 

45 

113 

135 

275 

50 

122 

140 

284 

55 

131 

145 

293 

60 

140 

150 

302 

65 

149 

155 

311 

70 

158 

160 

320 

75 

167 

165 

329 

INDEX 


Angle  for  setting  brushes,  333 
Acid  fumes  and  gases,  troubles  due 

to,  377 
Alternating      current      machines, 

causes    and    remedies  of 

troubles  in,  386 
Alternating-current  windings   (see 

also  table  of  contents  for 

Chapter  II),  26 
coils  for,  34 
coil  pitch  of,  32 
connections  for  coils  of,  43 
connecting  a  chain  winding,  48 
changing  star  to  delta,  41 
checking  phase  relationship  in, 

52 
double-layer,    lap    connected, 

47 
distributed  and  concentrated, 

27 
easily  remembered  rules  for, 

51 

full  and  fractional  pitches,  37 
grouping  of  coils  for,  35 
lap  and  wave,  28 
laying  out  and  connecting,  35 
phase  spread  of,  32 
polarity  of  coil  groups  in,  40 
progressive  and  retrogressive 

wave,  41 

simple  winding  diagram  for,  38 
single  and  polyphase,  30 
spiral  or  chain,  27 
2-phase  from  4-phase,  33 
3-phase  from  6-phase,  33 
types  of,  27 

whole  and  half-coiled,  30 
Armature  slots,  23 


Armature  winder's  tools,  444 
Auto-starter,  overhauling  of,   351 


Balancing  armatures 

large  d.  c.  sizes,  87 

machine  for,  460 

small  and  medium  sizes,  150 
Banding  procedure: 

for  small  d.  c.  armatures,  65 

for  large  d.  c.  armatures,  82 

for  railway  armatures,  99 

for  large  rotary  armature,  410 

for   different   armatures,    146 

precautions,  148 

simple  scheme,  408 

size  of  wire  for  bands,  148 

tension  of  bands,  147 

tool  for  applying  large  bands, 
86 

use  of  crane  for,  409 
Banding  wire: 

for  large  armatures,  84 

for  all  sizes  of  armatures,  146 

knock  in  armature  caused  by, 
395 

tension  of,  147 

tension  block,  454 
Bar  bender  for  making  coils,  458 
Bars  of  commutator : 

boring  out,  313 

copper  for,  319 

end  rings  for,  316 

excessive  wear  of,  318 

high  and  low,  302 

removal  of,  304 

repairing  of,  305 

repair  of  burned  sections,  303 

replacing  of,  306 

tightening,  306 


501 


502 


INDEX 


Basket  coils  for  induction  motors, 

195 
Belts,  poor  joints  in  cause  sparking 

of  brushes,  394 
horsepower    transmitted    by, 

491 

pulley  sizes  for,  492 
rules  for  use  of,  492 
speed  of  pulleys  for,  492 
static  sparks  from,  440 
troubles  due  to  tension  of,  391 
Bench    stand   for    winding    d.    c. 

armatures,  447 
Blackening  of  commutator: 

remedy  for,  343 

Boring  out  end  of  commutator,  313 
Bracing  windings  of  turbo-genera- 
tors, 228 

of  large  a.  c.  generators,  223 
Break-down  test  for  turbo-genera- 
tors, 229 

Bridging    commutator    bars,    258 

Brushes,  adjusting  and  correcting 

troubles    (see   also   table 

of    contents  for  Chapter 

XIII),  326 

adjustment  of  brush-holders, 

329 

angle  for  setting  of,  333 
blackening  of,  343 
causes  of  rapid  wear  of,  331 
causes      and      remedies      of 

troubles,  341 
checking  setting,  334 
chattering  of,  remedy  for,  344 
contact  drop,  334 
current  density,  335 
effect  of  defective  fields,  339 
fitting  or  grinding-in,  326 
friction  of,  335 
glowing,  335 
hardness,  336 
heating  of,  remedy  for,  343 
honey  combing,  335 
incorrect  thickness  of,  340 
incorrect  spacing,  338 
inertia  of,  336 


Brushes  (cont.): 

locating  causes  of  troubles,  337 

locating  electrical  neutral  for, 
332 

picking  up  copper,  343 

pressure  of,  334 

specific  resistance,  335 

terms  used,  334 

too  low  brush  pressure,   337 

unequal  air  gaps,  339 

wrong  characteristics  of,  341 
Brushes  for  undercut  commutator, 

322 

Brush-holder,  adjustment  of  reac- 
tion type,  406 

heating  of,  407 

making  adjustments  of,  329 
Brushes  set  incorrectly,  404 

wrong  setting  of,  405 
Brush  shunts,  loosening  of,  344 
Brush  studs,  heating  of,  403 
Burned  out  starting  winding,  429 
Burred  commutator  bars,  troubles 
resulting  from,  402 


Calculating    wiring    circuits,    for- 
mula for,  468 

Centigrade  and  Fahrenheit  tem- 
peratures, equivalent 
values,  499 

Chain  winding,  connecting  of,  48 
Chain  drives: 

calculating  size  of  chain,  493 
data  for,  494 

pinions  and  sprockets  for,  403 

Changes  in  d.  c.  motors  (see  also 

table     of     contents     for 

Chapter  X),  237 

Changes  in  speed  of  d.  c.  motor, 

237 

air  gaps,  242 
brushes,  243 
connections  to  commutator, 

243 

operating  voltage,  237 
winding  wave  to  lap,  248 


INDEX 


503 


Changes  in  duplex  wave  winding, 

252 

connections  of  coils,  244,  253 
Changes  in  induction  motor  wind- 
ings (see  heading,  proced- 
ure when  reconnecting  in- 
duction motors): 
factors  limiting,  286 
practical  methods  for,  261 
reconnections  frequently 

made,  272 
Changing  440  volt  motor  for  220 

volt  operation,  435 
Changing  star  to  delta  connection, 

41 

Characteristics    of    brushes,    341 
Chord  factor,  meaning  of,  269 
Circuit-breakers   for  motors,    489 
Cleaning  motors  with  compressed 

air,  420 
troubles     due     to     improper 

cleaning,  376 
Cleaning  slots,  solutions  for,   140 

by  filing,  140 
Coil  throw,  7,  70 
Coil  and  commutator  insulation, 

test  voltage  for,  175 
Coils: 

applying    insulation    on,    163 
connections  with  dead  coils,  98 
classification  of,  5 
for  large  d.  c.  armatures,  77 
for   railway,    mill    and   crane 

motors,  90 

for  a.  c.  windings,  141 
forms  for  making,  140 
inserting  in  open  slots,  69 
inserting  threaded-in  type,  60 
insertion  in  railway  armatures, 

94 

insertion  in  large  d.  c.  arma- 
tures, 81 

insulating  for  open  slots,  67 
insulation  for  different  volt- 
ages, 164,  165,  167,  168 
insulation  for  d.  c.  coils,  164, 
165,  168 


Coils  (cont.): 

r  insulation  for  induction  motor 

coils,  164,  167,  170 
kinds  of  insulation  for,  153 
mica  insulation  for,  173 
pitches  of  d.  c.,  7 
pitches  of  a.  c.,  37 
placing  on  armature,  110 
removing  from  d.  c.  armature, 

57 

repairing  damages  to,  174 
shaping    end   connections,  70 
table  for  connecting,  110 
test  voltage  for  insulation  of, 

175 

thickness  of  insulation  for,  163 
types  and  shapes  of,  3 
wire,  strap  and  bar  wound,  34 
Coil  raising  tool,  449 
spreader,  457 
taping  machine,  457 
taping  needle,  449 
winding  device,  452 
Commutator: 
action  of,  1 

blackening  of,  remedy  for,  343 
bridging  bars  of,  258 
compound  for  plugging  of,  304 
connections  for  lap  winding, 

102 

connections   for   wave  wind- 
ing, 104 

flat  spots  on,  remedy  for,  342 
improvised   method   to   turn, 

411 

insulation  of  connections  to,  76 
locating    first    connection    to 

for  lap  winding,  101 
locating    first    connection    to 
for    wave    winding,    105 
making  connections  to,  101 
number  of  bars,  24 
plugging  of,  395 
poor    soldering,    troubles    re- 
sulting, 396 

reconnecting  with   dead  coil, 
252 


504 


INDEX 


Commutator  (cont.): 

reconnecting      duplex      wave 

windings,  252 
repairs  to,  301 

repaired  under  difficulties,  397 
sandpapering,  holder  for,  400 
seasoning  and  grinding,  148 
soldering  coil  leads  to,  144 
sparking  at  brushes  of,  341 
table  for  connecting  coils  to, 

111,  246 

trouble  from  burred  bars,  402 
troubles  in,  301 
undercutting  of,  320 
undercutting  with  electric  drill, 

400 

voltage  between  bars  of,  24 
winding  pitch  for,  6 
Commutator,  repairs  to  (see  also 
table  of  contents  to  Chap- 
ter XII),  301 
baking  of,  307 
blackening  of  in  spots,  320 
boring  out  end  of,  313 
brushes  for  undercut  commu- 
tator, 322 

burn  out  between  bars,  303 
causes  of  excessive  wear,  318 
causes  of  troubles  in,  301 
copper  for  bars,  319 
finishing  undercut  slots,   322 
making    micanite    end    rings, 

318 

mica  used  in,  315 
micanite  insulation  for,  317 
precautions  when  tightening, 

317 

refilling,  311 
remedies    for    high    and    low 

bars,  302 

removing  bars  and  mica,  304 
removing  grounds,  308 
repairing  burned  bar,  305 
replacing  burned  bar,  306 
shaping  mica  end  rings,   316 
templet     for     making     mica 
rings,  316 


Commutator,  repairs  to  (cont.) : 
temporary  cover  for,  310 
test   for   oil   saturated   mica, 

320 

tightening  up  bars,  306 
tools  for  undercutting   mica. 

320 

troubles  from  high  mica,  301 
turning  down  surface  of,  309 
undercutting  mica,  320 
Compressed  air  pressure  in  clean- 
ing motors,  420 

Concentrated    a.    c.    winding,    27 
Chain    or    spiral    a.    c.    winding, 

27 

Conductor  or  inductor,  3 
Connecting  coils  of  d.  c.  armature 

in  parallel,  244 
Connecting  coils  of  d.  c.  armature 

in  series,  253 

Connections  to  commutator  (see 
table  of  contents  for 
Chapter  IV). 

Contact  drops  for  brushes,  334 
Controller,  drum  type,  over  haul- 
ing of,  355 
Cost  of  making  repairs,  346 

estimating  of,  347 
Cutting  out  coils  of  d.   c.   wave 

winding,  109 

of  induction  motor  winding, 
288 

D 

Dead  coils : 

connections  of  coils  with,  98 
wave  winding  with,  108 
Device  for  detecting  faults  in  a.  c. 

windings,  232 

Diagnosis  of  motor  and  generator 

troubles    (see  also   table 

of  contents  for    Chapter 

XV),  376 

Diagrams  for  connecting  induction 

motor  windings,  292 
combination  scheme  for 
several  types,  436 


INDEX 


505 


Diagrams  for  change  of  connec- 
tions of  induction  motor 
windings,  262 

different  poles  and  phases, 
295 

2-phase  motors,  267 

3-phase  motors,  265 
Diamond  coils  for  induction  mo- 
tors, 198 

Direct-current  armatures  (see  also 
table,  of  contents  for 
Chapter  III) : 

balancing  large  sizes,  87 

balancing  small  and  medium 
sizes,  150 

banding  of,  65 

cleaning  slots  of,  140 

floor  stand  for,  75 

loop  winding  for,  63 

repair  shop  methods  for  re- 
winding of,  56 

six  steps   in   winding   of,    72 

winding  procedure  for  arma- 
tures having  partially 
closed  slots,  60 

winding  procedure  for  open 
slots,  66 

winding  procedure  for  large 
armatures,  77 

winding  procedure  for  railway 
and  crane  armatures,  90 
Direct-current  generators : 

speeds  and  poles  of,  25 

winding  procedure  for  3-wire 
design,  89 

winding  procedure  for  rotary, 

88 

Direct-current  machines,  causes 
and  remedies  of  troubles 
in,  376 

Direct-current  motors  changed  for 
new  operating  conditions, 
237 

adjusting  air  gap,  242 

brush  changes  for  operation 
at  lower  voltage,  243 

changes  in  speed,  237 


Direct-current  motors  (cont.) : 
changes  in  operating  voltage, 

238 

generator  used  as  motor,  241 
motor  used  as  generator,  241 
operating  on  one-half  or 

double  voltage,  239 
reconnecting      for      different 

voltage,  243 

size  of  wire  for  coils,  240 
speed  when  reconnecting  wave 

to  lap,  242 

Direct-current  windings  (see  also 
table  of  contents  for 
Chapter  I),  1 

classification  of  coils  for,  5 
equipotential    connectors,    21 
for  different  number  of  poles, 

21 

for  large  armatures,  77 
for  rotary  converters,  88 
for  3-wire  generators,  89 
for   railway,    mill    and   crane 

motors,  90 
formulas  for  lap,  11 
formulas  for  wave,  17 
lap  (multiple  or  parallel),  9 
lap  and  wave  for  large  arma- 
tures, 79 
loop  windings,  63 
multiplex  lap,  14 
multiplex  wave,  19 
numbering  coil  sides  in  slots,  8 
parts  and  terms  of,  2 
symbols  used  in  formulas  for,  8 
symmetrical,  20 
testing  of,  122 
types  of,  2 

wave  (series  or  two-circuit),  16 
Dismantling  a  d.  c.  armature,  pro- 
cedure for,  56 

Distributed  a.  c.  winding,  27 
Drifts  used  by  armature  winders, 

448 

Drying  out  insulation: 
of  d.  c.  machines,  181 
synchronous  motors,  182 


506 


INDEX 


Drying   out    insulation  of  induc- 
tion motors,  183 
insulation     test     while,      184 
insulation     resistance     while, 

185 

Duplex  wave  winding,   reconnec- 
tion  of,  252 

E 

Electrical  neutral,  location  of,  332 
Element  of  d.  c.  winding,  3 
End  connections  of  coils : 

appearance  in  lap  and  wave 

d.  c.  windings,  10 
insulation  between,  63 
shaping  of  in  d.  c.  winding,  70 
thickness  of  insulation  for,  172 
End   rings   for    commutator,    316 
End  play  of  armature,  418 
Engine  type  generator,  inspection 

of,  371 
Equalizer  rings  for  d.  c.  windings, 

22 
use  on  multipolar   machines, 

120 

Equipotential  connectors  or  equal- 
izers, 21 
Estimating  cost  of  repairs,  347 


Failure  of  motor  to  start,  430,  432 
Field  coils : 

ad  jus  table  shunt  for,  416 

heating  of,  414 

insulation  of,  176 

test  for  dead  coils,  136 

test  for  reversed  connections, 

136 
Field  current  in  d.  c.  generators, 

473 
Flat  spots  on  commutator,  remedy 

for,  342 
Floor    stand    for    winding    d.    c. 

armatures,  75 

Form  for  recording  winding  data 
of  dismantled  armature, 
58 


Formulas    and    rules    for    d.    c. 

windings : 
for  calculating  wiring  circuits, 

468 

for  possible  symmetrical  wind- 
ings, 21 

lap  (multiple  or  parallel),  11 
multiplex  lap,  15 
multiplex  wave  (series-paral- 
lel), 19 

wave  (series  or  two  circuit),  17 
Frames  for  making  coils,  140 
Frequency    change,    reconnection 
of  induction  motor  for,  276 
effect  of  deer  ease  on  induction- 
motor-generator,  433 
Friction  of  brushes,  335 
Friction  cloth  blanket  for  commu- 
tator end  connections,  76 
Fuse  block  defective,  trouble  due 

to,  427 

Fuses,  sizes  for  a.  c.  motors,  442 
d.  c.  motors,  484 
causes  of  blowing  of,  389 
starting  a.  c.  motors,  487 


G 


Gears,  data  for,  496 
Generator,  engine  type,  overhaul- 
ing of,  371 

Generator  used  as  motor,  241 
Generators,  large  a.  c. : 

bar  and  connector  winding  for, 

217 
coils  for  partially  closed  slots, 

210 

coils  for  open  slots,  212 
connecting  coils  of,  221 
diamond    coils  for,  219 
double  windings  for,  221 
glowing  of  brushes,  335 
insulation  of  coils  for,  214 
inserting  shoved  through  coils, 

215 

lap  and  wave  connections  for, 
214 


INDEX 


507 


Generators,  large  a.  c.  (cont.): 
bracing  heavy  windings  of,  223 
testing  windings  of,  221 

Grinding  commutator,  148 

Grounds  in  d.  c.  windings: 
causes  of,  130 
locating  dead  grounds,  133 
telephone  receiver   test    for, 

135 
tests  for,  131 

Grouping  coils  of  induction  motor 

winding,  289 

odd  number  coils  per  group,  54 
procedure  for,  35 


Half-coiled  and  whole-coiled  a.  c. 

windings,  30 
Hand  tools  for  armature  winders, 

445 

Heating  of  brush  studs,  403 
Heating    of   motor   or    generator, 

remedies  for,  343 
caused  by  poor  soldering,  396 
caused   by   variation   of   fre- 
quency, 438 
hot  stator  coils,  391 
Holder  for  sand  papering  commu- 
tator, 400 
Honey   combing   of  brushes,    335 

remedies  for,  343 
Hoods  for  armatures,  99 

making  of,  145 
Hot  bearing,  relief  for,  440 


Impregnating  compounds  (see  also 

varnishes),  176 
Induction  motors: 

basket  coils  for,  195 

coil  insulation  for,   163,  167, 

170 

connecting  coils  of,  205 
diamond  coils  for,  198 
insulation    of  slots,  195 


Induction    motors  (cont.): 
inserting  new  coil  in,  203 
open  slots,  winding  of,  201 
operated  on  different  voltages 

and  frequencies,  284 
overhauling  of,  363 
partially  closed  slots,  winding 

of,  194 

phase  wound  secondary,  209 
procedure    when    connecting 

coils  of  new  winding,  288 
reconnecting    for    changes    in 

voltage,  frequency,  phase 

and  speed,  272 
squirrel  cage  secondary,  20/ 
testing  windings  of,  202 
2-phase  stator,  winding  of,  202 
winding  small  sizes,  192 
winding  with  basket  coils,  196 
winding   with  diamond  coils, 

199 

Inspection    and    overhauling    of: 
auto-starters,  351 
compound   d.    c.    motor,    358 
d.  c.  motor  starters,  349 
drum- type  controllers,  355 
engine  type  generator,  371 
induction  motor,  363 
single-phase  motor,  368 
slip-ring  motor,  366 
Inspection    and    repair    of   motor 

starters,  motors  and  gen- 
erators, 345 

cost  of  making  repairs,  346 
estimating  cost  of  repairs,  347 
Insulation  for: 

a.  c.  coils,  164,  167,  170 
coils  and  slots,  kinds  of,  156 
coils  used  in  open  slots,  67 
coils    of   railway    motors,    92 
commutator    connections,    76 
core  of  railway  armatures,  93 
d.  c.  coils,  164,  165,  168 
different  voltages,     163,     164, 

165,  166 

electrical  protection,  155,  156 
end  connections  of  coils,   63 


508  INDEX 

Insulation  for  (cont.) :  Insulating  materials  (cont.) : 

field  coils,  176  presspahn,  154 

formed  coils,  163  rawhide  fiber,  160 

high  temperatures,  155  ,    red  rope  paper,  oiled,  shel- 

large  d.  c.  armatures,  79  lacked,     and    varnished, 

mechanical  protection,  153  161 

open  slots,  69  shellacked  bond  paper,  1G1 

partially  closed  slots,  61  test  voltage  to  use,  175 

phase  coils,  172  varnishes   and  compounds, 

slots,  thickness  needed,  163  176 

Insulation  from  copper  to  copper,  vulcanized  fiber,  154 

between  coils,  172  Insulation    resistance,    measuring 

Insulating  materials:  of,  185 

treated  cloths,  156          \,' .  megger  test  for,  185 

cotton  tapes  and  cloth,  156  voltmeter  test  for,  184 
Empire  cloth,  156                       Interpole    motor,    checking    con- 

Kobak  cloth,  156  nections  of,  412 
oiled  canvas,  158 
oiled  cotton  drill,  158 
oiled  muslin,  158 

Japan  muslin   157  Knock    m    armature    cauged    b 

Japan  duck,  157  band  wireSj  3% 
varnished  cambric,  157 
•-"./       varnished  silk,  157 

pressboards,  fibres  and  papers,  L 

158 

asbestos   paper,    oiled    and      Lap  winding  (multiple  or  parallel) : 

varnished,  162  commutator  connections  for, 

drying  out,  181  102 

express    parchment    paper,  connecting    double  layer   for 

express  paper,  shellacked  a.  c.  machine,  47 

and  varnished,  161  for  d.  c.  armatures,  9 

fish  paper,  154  formulas  and  rules  for  d.  c. 

horn  fibre,  Japanned,  oiled,  armatures,  11 

shellacked  and  varnished,  grouping  coils  in  a.  c.  machine, 

159  35 

leatheroids,  160  multiplex,  formulas  for,  14 

manila  paper,  155  requirements  of,  103 

mica,  155  use  on  d.  c.   armatures,   115 

mica  for  coils,  173  use  in  a.  c.  machines,  116 

mica  paper  and  mica  cloth,  versus  wave,  112 

155  vs.  multiple  wave  (series-paral- 

micanite,  155  lei),  119 

micartafolium,  162  wave  and  lap  for  a.  c.  ma- 

pressboard,  oiled,  Japanned,  chines,  28 

shellacked  and  varnished,      Laying  out  and  connecting  a.  c. 

159  windings,  35 


INDEX 


509 


Lining  for  slots  (see  heading  of 
"insulation"  and  table  of 
contents  for  Chapter 
VII). 

Loop  winding  for  small  motors,  63 
illustrations  of,  64 

Loose  bearing,  trouble  due  to,  423 


M 


Machine     equipment     for    repair 

shop: 

banding  machine,  460 
bar  bender  for  coils,  458 
coil  spreader,  457 
coil  tapping  machine,  457 
coil    winder    for    lathe,    456 
combination  machine,  462 
equipment  needed,  444 
slotting  and  grinding  machine, 

459 

Making  new  coils,  141 
Megger  test  for  insulation  resis- 
tance, 185 
Mica  insulation: 

built  up,  164,  173 
for  armature  coils,  173 
Mistakes  and  faults  in  induction 

motor  windings : 
device  for  detecting  faults,  232 
grounds,  231 
improper  groups   connection, 

234 

open  circuits,  234 
reversal  of  coils  or  groups,  233 
short-circuits,  231 
use  of  wrong  number  of  coils, 

234 

wrong  number  of  poles,   235 
Motor  circuits,  wire  size  for,  473 
Motor  failing  to  start,  430 
Motor  reversed  direction  of  rota- 
tion at  high  speed,   412 
Motor-starter  overhauling  of,  349 
Motors,  safe  temperature  for,  415 
Motors  operated  on  double  volt- 
age, 419 


N 


Noise  in  three-phase  motor,   428 
Numbering  coil  sides  in  slots,  8 


O 


Odd    frequencies,    winding    small 

motors  for,  191 
Open   circuits   in   d.    c.  windings, 

causes  of,  127 
tests  for,  128 

telephone  receiver  test  for,  135 
transformer     testing     device, 

125 

Operations  before  and  after  winding 
d.  c.  armatures  (see  table 
of  contents  for  Chapter 
VI;.  139 

Overhauling  and  inspection  of: 
auto-starters,  351 
compound   d.   c.   motor,    358 
d.  c.    engine  type  generator, 

371 

d.  c.  motor  starters,  349 
drum- type  controllers,  355 
induction  motor,  363 
single-phase  motor,  368 
slip-ring  motor,  366 


Painting  windings,  151 
Phaoe    change,     reconnection    of 
induction  motor  for,  276 
Phase  coils,  insulation  of,  172 
placing  of,  291 
rearranging  in  induction  mo- 

*tor  winding,  278 
when     reconnecting     2-phase 

to  3-phase,  173 

Phase  insulation  when  reconnect- 
ing induction  motors,  271 
Phase  rotation,  testing  of,  420 

of  a.  c.  motor,  440 
Phase  spread  of  a.  c.  windings,  32 
Pinion  puller,  456 


510 


INDEX 


Pitchofd.c.  coils: 
definition  of,  6 
front  and  back,  7 
full  and  fractional,  7 
in  winding  spaces,  coil  sides 

and  slots,  7 

short-pitch  or  short  cord,  7 
Pitch  of  a.  c.  coils : 

effect  of  fractional  pitch,  38 
full  and  fractional,  37 
Plugging  commutator,  395 

filling  compound  for,  304 
Polarity  of  a.   c.   coil  groups,   40 
Pole-phase    groups    of    induction 
motor  winding,  connect- 
ing of,  290 
Polyphase   windings,    connections 

for,  43 

Potential  pitch,  23 
Pressure  for  brushes,  334 
Procedure  when  connecting  coils 
of  induction  motor  wind- 
ing: 
connecting  pole-phase-groups, 

290 

constructing   connecting    dia- 
grams, 292 
grouping  coils,  289 
number  poles  from  coil  throw, 

293 

placing  phase  coils,  291 
winding  diagrams  for  different 
number  poles  and  phase, 
295 

Procedure  when  reconnecting  in- 
duction motors,  272 
changes  in  voltage  only,  275 
change  in  phase  only,  276 
change  in  frequency,  278 
change  in  number  of  poles,  282 
change  in  speed,  273,  282 
change  in  resistance  of  squir- 
rel-cage rotor,  280 
constructing   connecting    dia- 
grams, 292 
cutting  out  coils,  288 
dead  ending  coils,  277 


Procedure      when      reconnecting 

(cont.) : 
double  circuit  star  to  single 

circuit-delta,  287 
effect  of  high  and  low  voltage 

on  operation,  283 
factors    limiting     change    in 

number  of  poles,  286 
grouping  coils,  289 
phase  and  voltage  change,  278 
points  to  consider,  272 
reconnections     frequently 

made,  272 

rearranging   phase  coils,    278 
Scott  or  T-connection,  2-phase 

to  3-phase,  269 

testing  reconnected  motor,  282 
winding  diagrams  for  different 

poles  and  phases,  2£5 
Procedure    when    winding    a.    c. 

machines : 
induction  motor  secondaries, 

207 

induction    motors    with    par- 
tially closed  slots,  194 
induction   motors   with  open 

slots,  201 

large  a.  c.  stators,  210 
repulsion-start  motors,  190 
small  single-phase  motors,  186 
small  polyphase  motors,    192 
stator   of  a.  c.  turbo-genera- 
tors, 223 

Procedure  when  winding  d.  c. 
machines  (see  also  head- 
ing direct  current  arma- 
tures) : 

banding  armatures,  146 
cleaning   armature  slots,    140 
connecting  coils  to  commuta- 
tor, 101 
cutting    out    coils    of    wave 

winding,  109 

insulating  coils,  164,  165,  168 
large  armatures,  77 
open  slots,  66 
partially  closed  slots,  60 


INDEX 


511 


Procedure  when  winding  (cont.) : 

railway,  mill  and  crane  arma- 
tures, 90 

rotary  converters,  88 

stripping  off  old  winding,  139 

3-wire  generators,  89 
Progressive  wave  winding,  107 

cutting  out  coils  of,  109 
Pulleys,  rules  for  size  of,  492 

speed  of,  492 

R 

Rating  of  a.  c.  generators,  440 
Reconnecting  d.  c.  winding: 

240  volts  lap  to  120  volts  lap, 

243 
240  volts  wave  to  120  volts  lap, 

248 
bridging     commutator     bars, 

258 

connecting  coils  in  series,  253 
connecting    coils    in   parallel, 

244 
reconnecting   with   one   dead 

coil,  252 
reconnecting  duplex  windings, 

252 
table  for   connecting  coils  to 

commutator,  246 
table  for  rewinding  coils,  245 
Reconnecting     induction     motors 
(see  also  table  of  contents 
for  Chapter  XI),  261 
changes   possible   in    existing 

winding,  272 

changes  frequently  made,  272 
change  in  voltage  only,   275 
change  in  phase  only,  276 
change  in  frequency,  278 
change  in  number  poles,  282 
change  in  speed,  273,  282 
change  in  resistance  of  squir- 
rel cage  rotor,  280 
cutting  out  coils,  288 
chord  factor,  meaning  of,  269 
diagrams  for  changes  in  con- 
nections, 263 


Reconnecting    induction    motors: 
factors     limiting     change    in 

number  of  poles,  286 
phase  and  voltage  change,  278 
phase  insulation,  271 
points  to  consider  before  recon- 
necting, 262 

rearranging    phase   coils,    278 
Scott  or  T-connection,  2-phase 

to  3-phase,  269 
table    of    possible    reconnec- 

tions,  266 

testing  reconnected  motor,  282 
with  dead  ended  coils,  277 
Reentrant,  definition  of,  14 
Relining  split  b  earings,  151 
Remedies   for    troubles    in    d.    c. 

machines,  380 
for  brush  troubles,  341 
Removing    old    coils    from    d.    c. 

armatures,  57 
Repairing : 

auto-starters,  351 

coils  damaged  while  winding 

armature,  174 
compound  d.  c.  motors,  358 
cost  of,  346 

d.  c.  engine  type  generator,  371 
d.  c.  motor  starters,  349 
drum-type  controllers,  355 
estimating  cost  of,  347 
induction  motor,  363 
single  phase  motor,  368 
slip  ring  motor,  366 
Repair  shop  equipment: 
banding  device,  86 
for  testing  d.  c.  windings,  138 
floor  stand  for  armatures,  75 
machines  and  tools,  444 
Repair  shop  methods  for  rewinding 
d.  c.  armatures  (see  also 
table     of     contents     for 
Chapter  III),  56 

Repair  shop  methods  for  rewinding 
a.  c.  machines  (see  also 
table  of  contents  for 
Chapter  VIII),  186 


512 


INDEX 


Repairs  to  commutator: 

baking  of  commutator,  307 
blackening     of     commutator, 

in  spots,  320 
boring  out  end  of,  313 
brushes  for  undercut  commu- 
tator, 322 

burn  out  between  bars,  303 
causes  of  excessive  wear,  318 
causes  of  trouble  in,  301 
copper  used  for  bars,  319 
finishing  undercut  slots,   322 
making    micanite    end   rings, 

318 

mica  used  in,  315 
micanite   insulation   for,    317 
precautions  when  tightening, 

317 

refilling  of  commutator,   311 
removing  bars  and  mica,  304 
removing  grounds,  308 
remedies  for  high  and  low  bars, 

302 

repairing  burned  bar,  305 
replacing  burned  bar,  306 
shaping  mica  end  rings,   316 
templet  for  making  mica  rings, 

316 

temporary  cover  for,  310 
test  for  oil  saturated  mica,  320 
tightening  up  bars,  306 
tools  for  undercutting  mica, 

320 

troubles  from  high  mica,  301 
turning  down  surface,  309 
undercutting  mica,  320 
under  difficulties,  397 
Retrogressive  wave  winding,  107 

cutting  out  coils  of,  109 
Reversal  of  speed  of  motor  while 

running,  412 

Reversed  coils  in  d.  c.  winding : 
cause  of,  131 

test  for  with  bar  magnet,  131 
test  for  with  compass,  132 
Ropes,  horsepower  transmitted  by, 

495 


Rotary  converters,  winding  of,  88 

banding  of,  410 
Rules  for  a.  c.  windings: 

arrangement  of  coils,  51 

checking*    phase   relationship, 
52 

connections  for  coils  of  differ- 
ent windings,  43 

grouping  coils,  35 

indicating     polarity     of     coil 
groups,  40 

reconnecting    induction    mo- 
tors, 434 
Rules  for  d.  c.  windings: 

for  possible  symmetrical  wind- 
ings, 21 

lap  (multiple  or  parallel),   11 

multiplex  lap,  15 

wave  (series  or  two  circuit),  17 

multiplex  wave  (series-paral- 
lel), 19 
Rules,  general,  498 


8 


Seasoning  and  grinding  commuta- 
tor, 148 
Shaping  insulating  cells  for  slots, 

448 
Single-phase  motors: 

connections  for  windings  of, 

191 

distribution  of  main  and  start- 
ing windings,  188 
inserting  skein  coils  in  slots, 

187 

insulation   for   slots   of,    186 
testing  windings  of,  192 
winding  of  by  hand,  190 
winding  for  odd  frequencies, 

191 
winding  repulsion  start  type, 

190 

winding  skein  coils  in,  186 
Single-phase    motor,    overhauling 

of,  368 
Single  vs.  multiple  windings,   118 


INDEX 


513 


Size  of  switches  for: 
d.  c.  motors,  484 
induction  motors,  487 
single-phase  motors,  486 
Shaping  mica  end  rings,  316 
Short  circuits  in  d.  c.  windings: 
causes  of,  122 
tests  for,  123 
transformer  testing  device  for, 

125 

telephone  receiver  test  for,  135 
Shoved  through  coils,  insertion  of, 

215 
Skein  coils  for  single-phase  motors, 

186 

Slings  for  armatures,  455 
Slip-ring  motor,  overhauling  of,  366 
sparking  at  slip-rings  of,  393 
Slots: 

cleaning  and  filing,  140 
inserting  coils  in,   61,  69,  81, 
94,    187,    196,    199,    202, 
203,  215,  227 

insulating  materials  for,  153 
insulation  at  ends  of,  172 
insulation  of  partially  closed 

d.  c.  type,  61 
insulation  of  open  d.  c.  type, 

69 

insulation  for  large  d.  c.  arma- 
tures, 79 
insulation  for  a.  c.  windings, 

186,  195,  214,  226 
numbering  of  coils  in,  8 
thickness  of  insulation  for,  163 
Slotting  and  grinding  machine,  459 
Small  motors,  loop  winding  for,  63 

winding  of,  64 

Solutions   for    cleaning  slots,    140 
Sparking  at  brushes,  remedy,  341 
due  to  poor  belt  joints,  394 
due  to  absence  of  balancing 

weights,  437 
Speed  change,  reconnecting  a.  c. 

motor  for,  273,  282 
Speeds  of  d.  c.  generators,  25 
safe  limit  of,  25 


Spiral  or  chain  a.  c.  winding,  27 
Split  bearings,  relining  of,  151 
Squirrel    cage    rotor,    change    in 

resistance  of,  280 
Stalling  of  wound  rotor  induction 

motor,  422 

Standard  motors  on  different  volt- 
ages and  frequencies,  284 
Static  sparks  from  belts,  440 
Stator  connections,  trouble  due  to, 

421 

Steel  shafting,   horsepower  trans- 
mitted by,  495 

Stripping  old  d.  c.  winding,  139 
Symmetrical  d.  c.  windings,  20 
Synchronous  motor  troubles,  388 
failure  to  start,  432 


Tables  for   winding   d.    c.    arma- 
tures, 110 

connecting   coils   to   commu- 
tator, 111 

changing  coil  connections,  246 
rewinding  coils,  245 

Tape  made  from  cotton  cloth,  181 

Temperature,  safe  for  motors,415 
CentigradeandFahrenheit,  499 

Terms  and  parts  of  d.  c.  winding,  2 
armature  coil,  4 
armature  conductor,  3 
concentric  coils,  5 
involute  coils,  5 
front  and  back  pitch,  7 
full  and  fractional  pitch,  7 
symbols  used,  8 
winding  element  or  section,  3 
winding  pitch  or  coil  pitch,  6 

Testing  d.  c.  windings  (see  table 
of  contents  for  Chapter  V) 
commutator,  136 
for   short    circuits,    123,    135 
for  open  circuits,  125,  128,  135 
for  grounds,  131,  133,  135 
for  reversed  coil,  131,  132 
for  reversed  or  dead  field  coils, 
136 


514 


INDEX 


Testing,  equipment  for,  138 

Testing  induction  motor  windings 

for: 

device  for  detecting  faults,  232 
exploring  with  a  compass,  236 
grounds,  231 
improper    group    connection, 

234 

open  circuits,  234 
order  in  making  tests,  235 
reversal  of  coils  or  groups,  233 
short-circuits,  231 
use  of  wrong  number  coils,  234 
wrong  number  of  poles,  235 

Threaded-in  coil,  60 

insertion  in  slots,  61 

Three-phase  motors,  reconnection 
for  (see  reconnection  of 
indue tion  motors),  261 

Three-phase  motors  on  single- 
phase  lines,  425 

Throw  of  coils,  7,  70 

Tightening  commutator  bars,  306 

Tools  used  by   armature  winder: 
band  wire  tension  block,  454 
banding  machine,  460 
bar  bender  for  coils,  458 
coil  raiser,  449 
coil  spreader,  457 
coil  taping  needle,  449 
coil  taping  machine,  457 
coil  winding  device,  452 
coil  winder  for  lathe,  456 
combination  machine,  462 
drifts,  448 

for  cutting  cell  lining,  448 
for  shaping  insulating  cells,  448 
hand  tools,  445 
leather  sling,  455 
made  from  hack  saw  blades, 

451 

pinion  puller,  456 
rope  sling,  455 
slotting  and  grinding  machine, 

459 

steadying  brace,  454 
wire  scraper,  449 


Transformer  rating  for  a.  c.  motors, 

478 
Troubles  in  a.  c.  machines,  causes 

and  remedies: 
due  to  electrical  faults,  388 
due    to    mechanical    adjust- 
ments, 387 
fuses  blowing,  389 
hot  stator  coils,  391 
in  windings  of,  387 
induction  motor  troubles,  386 
rotor  windings,  302 
sparking  at  slip  rings,  393 
stator  windings,  392 
synchronous  motors,  388 
tension  of  belts,  391 
testing  for  grounds,  390 
Troubles  in  d.  c.  motors  and  genera- 
tors, causes  of: 
electrical  defects,  379 
exposure  to   acid  fumes   and 

gases,  377 

lack  of  inspection  and  replace- 
ments, 378 

lack  of  proper  cleaning,   376 
operating     temperatures    too 

high,  378 

operation  in  damp  places,  377 
remedies  for,  380 
Troubles  in  commutator,  causes  of, 

307 
with    brushes,    remedies    for, 

341 

with  high  speed  motor,  417 
Turbo-generator : 

bracing  windings  of,  228 
break-down  test  for,  229 
coils  for,  223 
connecting  windings,  228 
inserting  coils  in,  227 
insulation  for  coils  of,  226 
forming  coils  for,  225 
testing  windings  of,  226 

U 

Undercutting  mica  with  standard 
tools,  320 


INDEX 


515 


Undercutting  mica  with  hand  tools, 

323 
with  portable  electric  drill,  400 


Varnishes  and  impregnating  com- 
pounds: 

baking  and  air-drying  varn- 
ishes, 178 

characteristics  of,  178 
classes  of,  176 

clear  and  black  varnishes,  177 
solvent  chart  for,  178 
table  of  uses,  177 

Voltage   change,    reconnection   of 
induction  motor  for,  275 
Voltmeter  test  for  insulation  re- 
sistance, 185 

Voltage  to  use  in  testing  insula- 
tion, 175 

W 

Wave  winding  (series  or  two-cir- 
cuit) : 

commutator  connections  for, 
104 

cutting  out  coils  of,  109 

dead  coils  in,  108 

duplex  winding,  252 

for  a.  c.  machines,  41 

for  d.  c.  armatures,  16 

formulas  and  rules  for  d.  c. 
armatures,  17 

grouping  coils  in  a.  c.  ma- 
chines, 35 

lap  and  wave  for  a.  c.  ma- 
chines, 28 

locating  first  connection  to 
commutator  for,  105 

multiple  wave  vs.  lap,  119 

multiplex,    formulas    for,     19 

progressive  and  retrogressive, 
107 

reconnecting  with  dead  coil, 
252 

reconnecting  240  volts  to  120 
volts,  248 

requirements  of,  107 


Wave  winding  (cont.): 

use  in  a.  c.  machines,  118 
use  on  d.  c.  armatures,   117 
versus  lap,  112 
Wedge  driver,  62 
Whole-coiled  and  half-coiled  a.  c. 

windings,  30 
Winding  d.  c.  machines: 

having  partially  closed  slots,  60 
having  open  slots,  66 
large  armatures,  77 
rotary  converters,  88 
3-wire  generators,  89 
railway,  mill  and  crane  motors, 

90 
Winding  a.  c.  machines: 

induction   motors   with   open 

slots,  201 

induction    motors    with    par- 
tially closed  slots,  194 
induction  motor  secondaries, 

207 

large  a.  c.  stators,  210 
repulsion   start   designs,    190 
single-phase  motors,  186 
small  polyphase  types,  192 
stator  of  a.  c.  turbo-genera- 
tors, 223 
Winding  data  needed  to  duplicate 

an  old  winding,  57 
Winding  diagrams  for  a.  c.  motors: 
developed  diagram,  compared 

with  circle  scheme,  53 
for  different  poles  and  phases, 

295 

simple  circle  diagram,  39 
Winding  parts  and  terms,  2 
Winding      procedure     for     d.     c. 
armatures    (see    heading 
"Direct   Current    Arma- 
tures")- 

Wire  scraper,  449 
Wire  size  for  motor  services,  473 
d.  c.  motors,  481 
motor  leads,  474 
3-phase  motors,  482 
Wire  table,  how  to  remember,  465 
Wire  gauges,  classification  of.  466 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 
BERKELEY 

Return  to  desk  from  which  borrowed. 
This  book  i 


MAY  6     1348 

MAR  1  Q  1950 


NOV  2  1  1952 
APR  1    1953^ 


MAY  6    1963 


V 


LD  21-100m-9,'47(A5702sl6)476 


OOUIO 


M184967 


Ub. 


THE  UNIVERSITY  OF  CALIFORNIA  UBRARY 


